You can find this exercise on page 3 of the book Synergistic Golf.
Exercise Type: PILATES
Exercise Category: ABDOMINALS, RHYTHM - TIMING, STRENGTH
Lie on your back with both knees hugged into chest and fingertips lightly behind ears with elbows open. Bend one knee toward chest while other leg extends out parallel with floor. Use your abdominals to keep a flat back to the floor and switch legs while rotating opposite elbow toward opposite knee. Repeat 10 times.