Retention should never be forced: when the body needs to breathe again, breathe. You know you have held the breath too long if there is a gasping sound upon release. Another sign is a feeling of heat in the breath as it releases. Retention has a psychological purpose: the mind is still when the breath is still. Enjoy these periods of quiet mind. There is also a physiological benefit to retaining the breath after rapid breathing; it allows the blood chemistry to come back to normal.
When we breathe quickly, we increase the amount of oxygen in the blood, but we deplete the level of carbon dioxide. The body normally controls these two important levels through two sensing mechanisms,  one near the heart and the other in the brain. When we are low on oxygen, the heart sensor eventually notices this, and sends a signal to the body to speed up respiration.  If, however, the level of carbon dioxide is too low, the body also responds. A lowering of carbon dioxide affects the blood’s acid balance and causes the small blood vessels in the brain to constrict, reducing the amount of oxygen going to this vital area. The sensor in the brain signals the body to slow down the breath, allowing carbon dioxide to build up again. 
There are two kinds of retentions of the breath (kumbhaka): the retention of the breath when the lungs are full (antar kumbhaka) and the shorter retention of the breath when the lungs are empty (bahir kumbhaka). These are often accompanied by bandhas or bonds  to keep the energies in the torso of the body, where they can be most effective. On the antar kumbhaka, the chin is dropped and the chest raised to prevent energy from escaping through the throat.  At the same time the perineum is engaged  to prevent energy from escaping to the lower body. On the bahir kumbhaka, a third bandha is added to the previous two, forming the maha bandha (the great lock). This third bandha is performed by drawing the lower belly in, moving the navel to the spine, and lifting the abdomen up and under the ribs.  While the stomach is lifting up, consciously lower the diaphragm.
Antar kumbhaka is advised after bhastrika. While holding the chin lock, bring your attention to the ajna chakra, the third eye. After completing one round of twenty breaths and the kumbhaka, without concerns or difficulties, this can be repeated two more times. Again, if any straining is occurring, if irritation arises, a stitch develops stop.
- — Called chemoreceptors.
- — There is a lag before the body notices and responds to this drop in oxygen levels in the blood.
- — An interesting thing occurs at very high altitudes: if the body is not trained to live at high altitudes, the lungs don’t get enough oxygen. The heart’s chemoreceptor initially orders the body to increase respiration. However, carbon dioxide also is depleted at higher altitudes and by faster respiration. The brain’s chemoreceptor orders respiration to slow down, despite the lack of oxygen in the blood. The brain’s signals override the heart’s signals, and the breath continues to slow down, sometimes to a dangerously, or even deadly low level. In these circumstances, conscious control is required to get enough oxygen into the blood. More information on the physiological affects of pranayama can be found in David Coulter’s book Anatomy of Hatha Yoga, which every yoga teacher should not just read, but study.
- — Also called valves, locks, or bridges.
- — This chin lock is called “jalandhara bandha,” named after the Hathi Yogi sage Jalandhari.
- — This is called “mula bandha” or “root lock.”
- — This is called “uddiyana bandha,” which means the upward flying bridge.