You can find this exercise on page 257 of the book Synergistic Golf.
Exercise Type: TOWEL STRETCH
Exercise Category: FLEXIBILITY, LEGS - FEET, LOW BACK
Lie on back with left leg outstretched on floor. Extend right leg straight to ceiling with wrapped towel around foot. Hold onto ends of towel with left hand only while right arm extends shoulder height to right on floor for support. Slowly pull ends of towel toward left side of body allowing leg to drop across torso and rest on floor. A stretch will be felt across outside of right hip and low back. Pause slightly then return leg toward ceiling. Repeat 5 times with same leg then switch. Do not force a stretch. Focus on exhaling during the exertion of the stretch.