Thanks for posting your questions here on the forum. Let's start with Yin Yoga and pregnancy. This topic has been discussed several times in the forum, so let me refer you to these posts: they answer your question and a lot more, plus you'll hear from women who have tried and tested Yin Yoga while pregnant, and afterwards.
http://www.yinyoga.com/forums/viewtopic.php?t=346
http://www.yinyoga.com/forums/viewtopic.php?t=357
http://www.yinyoga.com/forums/viewtopic.php?t=67
http://www.yinyoga.com/forums/viewtopic.php?t=443
http://www.yinyoga.com/forums/viewtopic.php?t=360
Now onto your other questions about, is it tension I am feeling or compression? The hip is a very vague area: there is so much going on there. Muscles, ligaments, joint capsules, ... how do I know what I am feeling?
Good question. Generally, we feel tension in the direction away from movement. For example, in a much simpler area of the body, if you extend your arm and you have very tight biceps, you will feel a stretch in the biceps. However, if you reach a point of compression this is generally felt in the direction of the movement. Imagine now that you are flexing your arm but you can't bring your wrist to your shoulder because you bicep, being flexed, stops your forearm. The muscles hitting muscles create compression.
Compression is generally felt in the direction of movement and tension in the direction away from the movement. So, let's see what this feels like in the hips.
If we are extending the hip, say by lifting the leg backwards such as in the balancing King Dancer pose or a lying down Shalambasana, compression will be felt in the buttocks or lower back. Tension, if that is what is stopping you, would be felt in the hip flexors or quadriceps.
If we are flexing the hip, lifting the knee towards the chest, tension may arise when the knee hits the chest, or, quite commonly, when the top of the pelvis (the ASIS) hits the lower belly. For many students the ASIS protrude quite a bit creating an uncomfortable pinching (compression) there (for example - in Childs Pose, many students don't like their legs together; this can be gotten around by slightly abducting the knees). If tension is stopping us from flexing the hip, generally it is felt in the back of the legs; tight hamstrings will do it, which is why bending the knees allows us flex the hip more, because the hamstrings are relaxed.
Those were the easy two: what about tension and compression in external rotation? What stops us now? If it is tension, the opposite direction feels (not pain, as you said) a resistance or challenge (if you feel pain, please STOP!) This would be the internal rotator muscles such as the adductors, the gluteus minimus and maximus. If you feel these muscles talking to you, you may be being stopped by tension. Keep working and over time, you will lengthen these muscles. If you are reaching a point of compression, you may feel the sensations more deeply, in the hips socket itself. At this stage, you will probably not get much more range of motion in external rotation.
I would suggest you read this post, where I go deeper into this case:
http://www.yinyoga.com/forums/viewtopic.php?t=348
I will leave it to you, as your own exploration, to examine the case for internal rotation, abduction and adduction of the legs. Let me know if you have more questions.
Cheers
Bernie
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