You must do the asana with your soul. How can you do an asana with your soul? We can only do it with the organ of the body that is closest to the soul - the heart. So a virtuous asana is done from the heart and not from the head. Then you are not just doing it, but you are in it. Many people try to think their way into an asana, but you must instead feel your way into it through love and devotion
B.K.S. IyengarA scientist sets out to conquer nature through knowledge - external nature, external knowledge. By these means he may split the atom and achieve external power. A yogi sets out to explore his own internal nature, to penetrate the atom (atma) of being. He does not gain dominion over wide lands and restless seas, but over his own recalcitrant flesh and febrile mind.
B.K.S. IyengarThe 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. IyengarThe practice of yogasana for the sake of health, to keep fit, or to maintain flexibility is the external practice of yoga. While this is a legitimate place to begin, it is not the end. Even in simple asanas, one is experiencing the three levels of quest: the external quest, which brings firmness of the body; the internal quest, which brings steadiness of intelligence; and the innermost quest, which brings benevolence of spirit.
B.K.S. Iyengar