In brief, extend your spine as much as possible and "rest" or touch your head to the floor. To me it is most important to always focus on and bring awareness to the core; strengthen, refine, lift and extend from the core AND always maintain the integrity of the breath... Core and breath! Everything else will come over time. I remember one practice I had in Mysore when I was particularly tight and I was taking it easy on my hamstrings - OK, I was being lazy. Guruji came right up behind me, grabbed my hips pushed me down onto my head and said, "touch your head." So simply, "touch your head." I still think David's book is one of the best resources for all of us. Good choice!