This exercise is progress the single leg pull. It requires increased length in the front of the hip, and great stability in the pelvis.
To do this exercise you will need a mat. Start by lying on your belly with your legs lengthened long beneath you. Inhale as you reach your toes away from you, and then exhale as you bend both knees and arc your heels towards the ceiling. Inhale as you reach your legs away from you and then exhale again as you bend both knees. Repeat 6-8 times.
Things to think about
Bending both legs increases the chance that your pubic bone will move away from the floor, which will cause you to use the muscles of your low back instead of your hamstrings. Be sure that as you press your pubic bone into the ground that you are also lengthening the muscular and fascial connections of the front of the hip.
If your hamstrings cramp as you do this exercise stretch the quads and then try the exercise again.