I found this discussion on Yoga Alliance’s LinkedIn group. It draws on one of my favorite Sutras—one I use regularly when starting to teach a new class.
Stephen Parker • The most important point about prevention of injury is that you never push your capacity. In Yoga Sutras II.46 & 47 Patanjali describes asana as “steady and comfortable” and that comfort is achieved through prayatna-shaithilya, “relaxation of effort” and anantya-samapatti, “coalescence with infinitude,” a phrase implying entry into samadhi. (Samapatti is essentially synonymous with samadhi in the Samadhi-pada.) By practicing in a meditative and contemplative way, always remaining two steps short of your capacity, you never have to push your body; it’s capacity will naturally expand before your efforts without pushing. Needless to say, this model doesn’t well fit a class where one is trying to get through 15 postures in 60 minutes. Two postures is more like it.
In the early 1970’s my master, Swami Rama, said, “American yoga is all ha- yoga (energetic solar force). There is no -tha (contemplative lunar force)!” He made it his mission to try to put the -tha back in hatha, but, unfortunately, American hatha-yoga practice has continued to move in the other direction, away from meditative depth.