You can find this exercise on page 99 of the book Synergistic Golf.
Exercise Type: YOGA-STRETCH
Exercise Category: FLEXIBILITY, HIPS, LOW BACK
Sit on the floor with left leg extended and right leg bent so that right foot touches left thigh. Reach left hand forward and grasp left foot, ankle or shin as comfort permits. Hold stretched position for 20-30 seconds. Do not bounce. Switch legs and repeat on right leg.