When did Yin Yoga begin? Did any one person create it? How did it become so popular? To answer these questions, we have to go way back in time and work our way forward to the dawn of the 21st century.
There are many documents describing yoga; some were written hundreds and even thousands of years ago: the Hatha Yoga Pradipika, the Gheranda Samhita, the Yoga Sutra, and many more. However, none of these ancient texts were meant to be read alone. They all required the guidance of a guru, to ensure understanding. The books were used more like notes – shorthand reminders of the real teaching. Much of the real knowledge was deliberately kept hidden; only when the teacher felt the student was ready was the knowledge revealed. We cannot tell simply from reading these old texts how the physical practice of yoga was performed. What we can say, with some certainty, is that the purpose of the physical practice was to prepare the student for the deeper practices of meditation.
In the earliest spiritual books of India, the Vedas, yoga is not described as a path to liberation, and asana practice is not described at all. Rather, yoga, among its many other meanings, meant discipline, and the closest word to asana was asundi, which described a block upon which one sat in order to meditate. By the time the Yoga Sutra was compiled,  yoga was defined as a psycho-spiritual practice aimed at ultimate liberation. Asana, however, was still a very minor aspect of the practice. The Yoga Sutra mentions asana only twice  in all one hundred and ninety-six aphorisms. And all that is said about asana is that it should be sthira and sukham: steady and comfortable. These are very much yin qualities, compared to the style of asana we see performed today in yoga classes. When we are still and the mind undistracted by bodily sensation, meditation can arise.
The point of yoga practices is to enter into a meditative state from which realization or liberation may arise. Different schools of yoga have different techniques for achieving this. Some even claim that one cannot become liberated while in the body. The goal in these dualistic schools is to get out of the body as fast as possible, but this must be done in the right way. Other schools rejected that approach and suggested, since we can meditate and practice yoga only while in the body, we must treat the body well. The body must be healthy. The focus of the Hatha Yoga schools was to build a strong, healthy body that would allow the yogi to meditate for many hours each day. In Hatha Yoga, the practice of asana began to take on a new, broader importance. However, the ultimate goal was still to be able to sit comfortably and steadily for hours.
The Hatha Yoga Pradipika (HYP) was written sometime in the 14th or 15th centuries C.E. by Swami Swatmarama. It is almost twice as long as the Yoga Sutra and has generated a lot of commentary since its writing. It is one of the oldest extant documents we have describing Hatha Yoga. Compared to today’s practices, however, it too has very little asana practice in it. More time is spent in the text describing the practice of pranayama and mudras than asana. In the section discussing asanas, there are only fifteen described, and of these, most are done while sitting on the floor. These are quite yin-like in their nature; however, a few postures are very definitely yang-like. The peacock (mayurasana) is prescribed, and if you have seen this posture performed, there is nothing relaxing or yin-like about it. We are told that one of the postures is supreme: once one has mastered siddhasana, all the other postures are useless.  Siddhasana is a simple, yin-like seated posture. The complete list of asanas described in the Pradipika are
Paschima tana ** (Today we would call this Paschimottanasa)
Siddhasana (also called Vajrasana) *
Bhadrasana (also called Goraksasana) *
* probably seated postures
** other postures intended to be held for a while
We are not given exact descriptions of each pose so it can be debated over which ones are meant to be held for long periods of time, or even which ones we can characterize as seated poses. But, only four postures can definitely be described as yang-like asanas: Kukutasana, Uttana Kurmasana, Dhanurasana, and Mayurasana. The other 11 were probably yin-like postures. To be complete, there were other postures described in the Hatha Yoga Pradipika, but they are not listed under the asana section of the book, but are given as basis for performing mudras or pranayama. These include the Maha Mudra posture (similar to Janu Sirsasana), and Viparita Karani, which is not described in any detail. So, coming up with an exact number of postures offered in the Pradipika depends upon your definition of asana, but clearly most of the postures were yin-like.
The Hatha Yoga Pradipika claims that Lord Shiva taught the Hatha Yoga sage Matsyendra eighty-four asanas.  Other myths claim there are eighty-four thousand or even eight hundred and forty thousand asanas. Regardless, only fifteen or so are listed in the Pradipika. And of asanas it is said that these should be practiced to gain steady posture, health, and lightness of the body.  Not mentioned in any of the Hatha texts is how long one should hold the pose. This is where the guru’s guidance is necessary. However, one can assume that the seated postures were meant to be held a long time while the more vigorous poses like the peacock were held for briefer periods. It is in the seated postures that the vayus (the winds or the breath) become trained through pranayama.  The Lotus Pose (padmasana) is the prescribed pose for conducting pranayama. 
As time went on, later texts expanded the number of asanas explained. The Gheranda Samhita, written perhaps in the late 1600s, a few hundred years after the Pradipika, describes thirty-two asanas, of which one-third could be said to be yin-like and the others more yang-like. A trend had begun: more yang asanas compared to yin asanas. A few decades later, the Shiva Samhita listed eighty-four asanas. By the time of the British Raj, when England began to colonize Indian culture and change the school system, asanas were beginning to become blended with forms from the gymnasiums. Wrestling, gymnastics, martial arts and other exercises were cross-fertilizing the asana practice. By the end of the nineteenth century there were thousands of asanas. Krishnamacharya  said he knew around three thousand postures but that his guru, Ramamohan Brahmachari, knew eight thousand. The era of yang yoga was upon us.
This gradual, and then sudden, evolution of asana practice moved the practice away from the original yin style of holding seated poses for a long time as a preparation for the deeper practice of meditation to the more active yang style of building strength and health. One is not better than the other; they are simply different. To sit for long periods of time in deep, undisturbed meditation requires a body that is open and strong. This opening, especially in the hips and lower back, is developed through a dedicated yin practice. However, if one is in ill health or weak, it is very difficult to sit with focus. A yang practice helps to build a “diamond” body, readying the student for the rigors of advanced yoga practice.
As we move into the 20th century, we still find both yin-like postures and yang being taught in Hatha yoga classes. In his book, Light on Yoga, first published in 1966, B.K.S. Iyengar offers several postures that he advises be held for long periods of time. For example,
Maha mudra – “hold this pose … one to three minutes.” (page 147)
Paschimottanasa – “try to stay … from one to five minutes” (page 169)
Supta Virasana – “[hold] this pose 10 to 15 minutes.” (page 125)
For several other postures, the student was advised by Iyengar to “Stay in this pose as long as you can …” These were yin postures, although that Chinese term was obviously not used. While yin postures were and still are a part of the Hatha yoga practice, they remained a minor part and no one seemed to be teaching classes that were entirely yin. On the other hand, there were more and more classes offered that were entirely yang, except for the ending Shavasana: Ashtanga, Power Yoga, Vinyasa Flow Yoga, Hot Yoga, Kundalini and many other styles typify this yang emphasis.
We know that the use of yin postures in yoga is not new. It has been around since the beginning of the physical practice of yoga. Thus no one person can be given credit for inventing yin postures. There was a time when all yoga was yin-like, and perhaps the balance was too far in that direction. A rebalancing occurred when yang postures grew in prominence. However, as time went on, yoga practice became more and more yang-like. Nature desires balance – we could say she demands balance. If we don’t seek it out, she will impose it upon us eventually. Yoga could not continue to be more and more yang without someone finding a way to bring it back into balance.
In the last decade of the 20th century, two teachers did start to bring yin postures back into the prominence they once had in the yoga world: Paul Grilley and Sarah Powers. Paul was first exposed to instances of long held postures in 1989, while attending Paulie Zink’s Taoist Yoga classes. The history of the term, Taoist Yoga is also interesting. It is not a term that has been in use for very long. Its first occurrence was probably in a book called Taoist Yoga: Alchemy and Immortality, written by Lu K’uan Yu’s (Charles Luk; 1898-1978) , which he published in 1973. The practice of Taoist Yoga equates simply to Chi Kung, but by using the word “yoga” in the name, Lu K’uan Yu was able to leverage the growing popularity of yoga to help students become interested in Chi Kung. After Lu K’uan Yu, a few other teachers, like Mantak Chia in the 1980’s, also began to refer to their Chi Kung as Taoist Yoga as well. (Paulie Zink, in his web site, made the association explicit when he wrote “Taoist Yoga is a form of Chi Kung …” .) On the other hand, many Daoist scholar, such as Louis Komjathy, Ph.D., Religious Studies, Boston University, believe the term Taoist Yoga to be a misnomer, because there is no yoga to be found in Daoist philosophy. In their view … “some so-called “Taoist Yoga” is a modification of Indian practices, while other versions derive from Chinese Wushu and Gongfu (Kung-fu) training exercises. As such, “Taoist Yoga” is part of what may be referred to as Popular Western Taoism, a form of New Age hybrid spirituality that appropriates some aspects of the religious tradition which is Daoism in order to increase cultural capital and marketability.”. 
Regardless of the historicity or authenticity of Taoist Yoga, it was in the Taoist Yoga classes that Paul Grilley attended where Paulie Zink offered asana practice that included both yin postures and yang movements. While Paulie’s Taoist Yoga was interesting, what Paul really resonated with was the long held postures. He got the idea that an entire yoga class could be yin in nature without any yang postures at all: an entire Yin Yoga practice was possible. He started offering this “all-yin” practice to his own Hatha Yoga students, and they too started to resonate with the practice. This was a new way to look at a very old paradigm. Unlike Restorative Yoga, which can also be viewed as yin-like, Paul’s Yin Yoga was designed for healthy students, not those recovering from an illness or pathology. Paul Grilley’s modern Yin Yoga was not easy: it was challenging. It took people outside their normal comfort zone, just as yoga had always done.
Sarah Powers loved the yin practice that she learned from Paul Grilley, but until he started to explain why the yin postures were so beneficial, she didn’t really incorporate it fully into her regular practice. Once Paul had worked out the physiological and energetic pathways to health and wholeness that Yin Yoga was utilizing, and once he started communicating these processes and benefits to his students, Sarah became convinced that Yin Yoga had to be an essential part of her own practice. She began to share her understanding with her own students. They in turn thirsted to learn more.
Sarah was the one who coined the term “Yin Yoga.” Until then, Paul had been referring to his practice as Taoist Yoga, because that is what Paulie Zink had called his practice. But Sarah pointed out that what she and Paul were offering was not the Taoist practice taught by Paulie, but a revamped subset of his teaching. They were offering only the yin postures, so the practice should be called Yin Yoga.
Sarah was already a renowned yoga teacher when she studied with Paul, and she was a teacher who travelled widely. As she traveled, she introduced students to the yin-side of yoga. Sarah also eloquently added explanations of the meditative qualities and benefits of the practice. In time, her classes came to include dharma discussions of Buddhist philosophy and psychology. Those who wanted to learn more about Yin Yoga, she directed to Paul. Soon, Paul was fielding requests from many students in many locations who wanted to learn more about the practice of Yin Yoga and its benefits. He began to write articles about Yin Yoga, and in 2002 published his book, Yin Yoga. At that time, Sarah created a VHS video called Yin Yoga and a CD recording of the same name. In time, both Paul and Sarah published DVDs explaining the yin-side of yoga.
Sarah’s and Paul’s students eventually became teaches in their own right. Yin Yoga began to establish an open source lineage through these teachers, and the practice continued to spread: a critical mass was building – one that would soon bring the yin-side of yoga into thousands of yoga studios throughout the world.
- — Scholars are not sure exactly when this book was compiled, but probably between 300 c.e. to 400 c.e.
- — Yoga Sutra, II-29 and II-46.
- — Hatha Yoga Pradipika, I-43.
- — Ibid, I-35.
- — Ibid, I-19.
- — Pranayama is one way to harness the inner energies we work with in yoga. This is described in more detail in Chapter Five: The Energy Body.
- — Ibid, II-7 and 8.
- — Whose students included BKS Iyengar, Pattabhi Jois and TKV Desikachar.
- — See Paulie Zinks web site as it was in March 4, 2003.
- — The original source of this quotation is no longer available, but for a similar quotation, see Common misconceptions concerning Daoism (Taoism)