From Light on Yoga by BKS Iyengar
The asana tones the abdominal organs and relieves backaches. Ladies suffering from severe pain in the back during the menstrual period will obtain relief in this pose and the back will be soothed.
*There are two different techniques for doing this asana which are given in Light on Yoga.
Squat on the haunches with the feet together. The soles and the heels should rest completely on the floor. Raise the seat from the floor and balance.
Now widen the knees and move the trunk forward until the armpits extend beyond the knees.
Bend forward and catch the back part of the ankles.
After gripping the ankles, exhale and move the head down to the toes and rest the forehead on them.
Maintain the pose for about a minute breathing normally.
Inhale, raise the head, release the ankle grip and relax on the floor.