You can find this exercise on page 25 of the book Synergistic Golf.
Exercise Type: TOWEL
Exercise Category: FLEXIBILITY, LEGS - FEET
Lie on back with right leg extended toward sky. Wrap towel around bottom of foot and hold ends of towel in right hand only. Left arm extends out to left for support while left leg stays straight on floor. Lower right leg 90 degrees straight out to right side keeping leg straight. Stretch will be felt down inside of leg. Exhale while reaching through stretch. Return back to start. Repeat 5x one leg then switch. Challenge: Keep hips stable on floor while stretching leg.