My Pilates teacher taught me this. It is a gentle Feldenkrais exercise to help mobility of the lower back.
The intention here is to improve the ability for your pelvis to move separately from your ribcage and thus improving mobility of your waist area.
Follow my instructions and slowly feel which side of your waist and even thigh feels tightest. Dont necessarily feel for discomfort as it can be misleading. Look at how far your knees can move side to side, how much of your body is pulled in that direction. You should be able to rock your knees to the side without your shoulder moving. I.e if when swinging your knees left, right shoulder should stay comfortable and flat not lifting.
When you lie on your side and rock the top knee forward. Be sure to stabilise your ribs with you arm so they stay still in the exercise. It is a common mistake to allow your ribcage to roll with the moving knee. Again be slow and look to see how far forward and back your knee can move. Work gently until the movement becomes the same and then swap to do the other side.
Move slowly, not fast. Moving fast with only engage elasticity, moving slowly will start to change your tissue.
