B.K.S. Iyengar and the development of posture
Warrior one, B.K.S. Iyengar
First photo on the left, from his book "Light on Yoga" next photo, years later at a public demonstration.
Can you see the difference in postural integrity between these two expressions of the same pose by the same person?
In the first, he's using his considerable range of motion to accomplish the form of the pose - in the second, his foundation through the legs and pelvis is much stronger, consequentially the pose expresses ease and lightness.
His head position has also changed - in the first he is gazing toward his thumbs, neck folded back. In the second he is gazing forward, able to recruit the muscles of his shoulders and upper back.
Iyengar didn't have anyone teaching him the difference - he figured it out on his own - all the while teaching others as his own knowledge base expanded.
This is the imperfect world of teaching a developing discipline.