Finding Yoga Tag

I run a Yoga for Pain program for people with persistent pain.  Today I was inspired by the progress the current group have made in their understanding of their bodies and themselves. 16 weeks into their yoga experience they attempted - and many performed - crow pose, a challenging arm balance. To give you an idea of just how impressive this is, in lesson one of Yoga for Pain they moved only hands and feet because anything more would exacerbate their pain. What makes this kind of progress possible?