You can find this exercise on page 157 of the book Synergistic Golf.
Exercise Type: STRENGTH
Exercise Category: BACK, LOW BACK, POSTURE, STRENGTH
Lie face down on floor with fingertips behind ears to keep shoulders and elbows back. While keeping legs and toes on floor, slowly extend the spine (lift chest off floor) and rotate one elbow high in air while other elbow touches ground. Pause at the top of each movement before returning to center. Repeat both side 10x each.