Working on the hip joint

I discovered a little while ago that one could try and work on the hip joint (as with every other joint I suppose) from within the joint. The hip joint is a part of the pelvic girdle (made of several fused bones). The outer surface of the hip bone has a deep impression in which the almost spherical head of the thigh bone (femur) fits. The thigh bone is the longest and strongest bone of the body and it is held within the cavity of the hip bone by strong ligaments. This is a ball and socket joint, where the head of the thigh bone is like a ball that can move in the cavity (socket) of the hip bone. This kind of joint allows for a wide range of movement of the leg. That's the anatomy part of it.

I always used to think that this meant that one could move one's leg about the hip in different ways and the movement depended on the condition of the different muscles involved. But recently, during the Yoga practice, I have realized that when Masterji says, "Open the hip joint," one can actually attempt to loosen 'something' inside (ligaments or more muscles around, or both perhaps?) such that the joint feels freer from within. The bones are not in such close proximity any more, the tightness is lessened and the range of movement increases on its own. As the hip joint is involved in so many asanas, for me, focussing on this aspect alters the movement in different postures, in different ways. In several asanas, I often used to determine the final position to be reached by observing the position of the knee joint. I feel now that the position of the knee is often just an indicator of the degree of movement that is being allowed from the hip joint. Pushing the knee further many times will help only to a small extent as the limitation often times is due to stiffness in the hip joint (very easily observable in asanas like Virabhadrasana, Baddhakonasana, Padmasana). Once this opens, the knee automatically moves further.