This is a motion rotating your waist while your leg is in a fixed position. Lay down sideways, fix your leg and pelvis, and rotate at the waist. When rotating the waist in this way, it not only stretches the waist but also stretches the shoulder chest and hip joints together. The spine muscles that help to rotate our upper body are generally connected from the rubs upwards and downwards to the pelvis. As you revolve your upper body with your pelvis fixed, the muscles along the spine become relaxed.