Downward facing dog is one of the most popular poses and identifiable poses in yoga. It behooves all yogis to ease into this asana from the start. With tight hamstrings, the pelvis tilts backwards (retroversion) and this pulls the lumbosacral fascia and back muscles, so that the lower back loses some of its natural arch. If you bend the knees this will effectively release the hamstrings and free the lower back. Contract the iliosoas and this will tilt the pelvic forward (anteversion). This action will bring back thighs. Now contract the triceps to straighten the elbows, contract the quads and straighten the knees and draw the hamstrings out to a full stretch to complete the pose.
~Mary Jane - www.fengshuiyoganj.com