This exercise is the classic Pilates Single Leg Pull Exercise. It strengthens the hamstrings in both knee flexion and hip extension.
To do this exercise you will need a mat. Start by lying on your belly with your legs lengthened beneath you. Place your elbows underneath your shoulders with your hands clasped together in the center. Inhale to prepare and exhale as you lift your belly and slide your shoulder blades down your back. Maintain this position as you inhale and then exhale as you bend one knee and bring your heel towards your pelvis. Inhale as you lengthen it away from you and exhale as you bend the other side. Repeat 6-8 times on each side.
Things to think about
This position will provide some support for the head neck and shoulders, but it is important also to lift the muscles and fascial tissue of the front of the spine to support each vertebra.
As you bend the knee be sure that the pubic bone is connecting evenly to the ground. If you feel your pelvis rolling from side to side, take some time to lengthen the front of the hips. And then be sure that you maintain that length as you bend the knee.