Full movement instruction is here at Yogasync.tv
The work through the feet and legs will stabilize and strengthen them. The hamstrings are lengthened, but safely so, because of the squeezing in action of the greater trochanter bones. When the head is more towards the ground, Uttanasana is a standing inversion so the brain will become quiet and calm.
Avoid this movement if have osteoarthritis of the knees, or diarrhoea, and in the later stages of pregnancy. If you suffer from Low Blood Pressure come out of the movement slowly.