You can find this exercise on page 145 of the book Synergistic Golf.
Exercise Type: YOGA-STRETCH
Exercise Category: FLEXIBILITY, LEGS - FEET
Lying face down, bend one knee bringing foot up towards the hip. Reach back with the arm on the same side and grasp the ankle or lower leg and pull the foot towards the hip until a gentle stretch is felt along the front of the hip and thigh. Keep abdominals pulled in and both hip bones on the ground. Hold each side 10 to 15 seconds.