You can find this exercise on page 293 of the book Synergistic Golf.
Exercise Type: PILATES
Exercise Category: ABDOMINALS, CLUB CONTROL - ACCURACY, POSTURE, STRENGTH
Start in pushup position with legs straight. Inhale lifting one foot off floor in flexed position. Exhale and further flex the supporting foot while pointing the foot that is lifted. Repeat 5x then return foot down and repeat other side. Focus on maintaining straight body alignment throughout entire movement.