Assume a kneeling position on all fours so that you are resting on your hands and knees. Arch your back like a cat and at the same time drop your head slowly and tuck in your pelvis.

Now reverse the exercise by gently forming a "U" in your back.

As you perform this movement, raise your head and stick your buttocks out at the same time. As with all exercises, this one in particular, do it slowly and smoothly. Do the exercise twice more. It's purpose is to stretch the back muscles to make them more flexible.