In this Yin practice we mostly lengthen the hamstrings with long holds, releasing tension through the whole back of the leg.
You should never feel sharp or painful sensations in the lower back. If you do, modify the posture by keeping the back flat. You still get the work, since the focus is the hamstrings, calves and Achilles tendon. Use the props as needed, and remember that feeling matters more than form in this practice.
