You can find this exercise on page 139 of the book Synergistic Golf.
Exercise Type: YOGA-STRETCH
Exercise Category: FLEXIBILITY, HIPS, LEGS - FEET, LOW BACK
Lie on your back. Bring one knee into the chest and hold for approximately 5 seconds. Release slightly and repeat 3x total. Switch legs. At the end, bring both legs into chest. Knees can be slightly apart as to not put too much stress on abdominal wall. Breathe slowly and allow body to give in to the stretch. Relax the head and neck.