The challenge of yoga is to go beyond our limits - within reason. We continually expand the frame of the mind by using the canvas of the body. It is as if you were to stretch a canvas more and create a larger surface for a painting. But we must respect the present form of our body. If you pull too much at once, we will rip the canvas. If the practice of today damages the practice of tomorrow, it is not correct practice.
B.K.S. IyengarTo do is hard, but to teach is still harder. Do not teach only to teach. Teach to improve the pupil. To be a teacher requires tremendous, vigorous discipline on oneself. We are teachers because somebody demands it from us. But the teacher should first rub his own self, and teach afterwards
B.K.S. Iyengar