Supta means lying down. Pada is the foot. Angustha means the big toe. This asana is done in three movements.
From Light on Yoga by B.K.S. Iyengar
The legs will develop properly by the practice of this asana. Persons suffering from sciatica and paralysis of the legs will derive great benefit from it. The blood is made to circulate in the legs and hips where the nerves are rejuvenated. The pose removes stiffness in the hip joints and prevents hernia. It can be practiced by both men and women.