You can find this exercise on page 71 of the book Synergistic Golf.
Exercise Type: PILATES
Exercise Category: ABDOMINALS, HIPS, RHYTHM - TIMING, STRENGTH
Lie on back with one leg straight on floor and other leg straight in air. Legs form 90degree angle. Arms at side palms down. While keeping entire body on floor still, rotate leg in air 10x one way then reverse. Slight pressure of internal muscles will be felt throughout body. Repeat other leg.