a. (preliminary movement) Standing, hands on hips, feet shoulder-width apart, lean the trunk backward and forward quickly.  In this exercise, do not bend the back or neck, nor bend at the waist. Rather, on the back lean, protrude the hips and abdomen, and on the front lean, stick out the buttocks. This is done fast enough that you should be doing many (4-8) such back and forths per breath, and the exercise should last four breaths.
b. (the rubbing) During backward motion, bring hands up the abdomen to the lower ribs and the around the side to the back, where on the forward lean, they move down the loins and buttocks. [N.B. Ladies should make the front-stroke higher, from under the breasts to the collarbone.]

(excerpt from fan summary)