This practice is so called because the legs move up and down like a butterfly’s wings, and when mastered it loosens the rigidity of the muscles and tendons in the groins which prevent the knees from touching the ground during meditative asanas.
Sit on the floor with the legs extended in front of the body. Bend the legs sideways and place the soles of both feet in contact with each other. Don’t strain, but try to slide the feet as near as possible towards the buttocks keeping the soles in contact.
Hold the feet with the two hands. Place both the elbows in contact with the inside tops of the thighs. Then gently push the knees of both legs towards the ground, using the elbows as levers. After a short duration release the legs and allow them to rise upwards. Again push the legs downwards. Without strain, try to push the knees as close as possible to the ground. Repeat this movement as often as available time will permit; up to 10 or 20 times is reasonable.
Release the hold of the hands on the feet. Keep the legs in the same position. Place the hands on the knees; some people will find this a little difficult, but try. Relax the legs as much as you can. Using the arms push the knees up and down.
Try not to utilize the leg muscles in this practice. Do this as many times as you want; 20 to 40 times is a reasonable number if you have the time.