Purva and paschima are the Sanskrit words for the east and west. In the context of the human body purva means the front side or the ventral side, paschima meaning the dorsal or back side. Tana means to stretch. Thus the name of this asana indicates that in it the back side is stretched. It is also called Paschimottana because the back side faces up (uttana). A third name often given to it is Ugrasana, ugra meaning fierce. This posture is highly talked about in the yoga texts. The description of it in the Shivasamhita as follows: “Keeping the legs straight in front of the body and holding the feet fast with the hands, the forehead is placed on the knees. This is called Paschimottana. One who masters it and practises it every day makes the vayu (air) flow through the dorsal path.”
Making the vayu flow through the dorsal path has a great significance in connection with the arousal of the dormant spiritual power called Kundalini. This asana is very important for that because of the stretching influence on the lower back where the Kundalini power is said to reside.
Sitting on the seat, the legs are kept straight in front of the body with the heels on the ground and toes pointing upward. The hands are kept on the thighs with the palms facing downward. The hands are then advanced slowly forward, at the same time bending the trunk forward, and without allowing the knees to be raised even slightly, the toes are grasped with the hands. The forehead is then lowered on the knees. Many people find it impossible to achieve this in the beginning because there is much strain on the hamstring muscles (on the inner side of the knees). But these muscles can be trained to bear the strain within two to three weeks by subjecting them to a steadily increasing stretch every day for a few seconds. If one fails to grasp the feet with the hands on the first day, one should take the hands as near the feet as possible, and remain in that state for a few seconds. The hands may then be brought back to the starting position, and after relaxing for a few seconds the procedure may be repeated again.
There should be no haste and no feeling of undue strain. When one starts grasping the feet with the hands without raising the knees, the head should be kept on the knees and the elbows should be kept on the ground by the two sides of the knees. The duration of the final pose may be increased gradually from a few seconds to begin with, to one minute. This posture exercises the muscles of the limbs and the back, especially the base of the back by stretching them, and improves the tone of the organs in the abdominal cavity. This influence on the abdominal viscera can be enhanced by contracting the anus and pelvis with a deep exhalation during the final pose, and hulding the breath out. This asana helps to reduce the fat accumulated on the belly.