You can find this exercise on page 229 of the book Synergistic Golf.
Exercise Type: TOWEL
Exercise Category: FLEXIBILITY, LEGS - FEET
Lie on back with legs outstretched. Hold onto ends of towel with both hands. Bend right leg into chest and wrap towel around bottom of lifted foot. Straighten right leg toward ceiling feeling a stretch down entire back of leg. Toes will flex toward body pushing heel toward sky. Pause slightly then return to bent. Repeat 5 times with same leg then switch. Do not force a stretch. Focus on exhaling during the exertion of the stretch.