You can find this exercise on page 211 of the book Synergistic Golf.
Exercise Type: TOWEL
Exercise Category: BALANCE, FLEXIBILITY, LEGS - FEET
Stand tall holding ends of towel in both hands. Balance on left foot while lifting right foot and placing it in middle of towel. Stand tall lifting right knee up toward chest. Maintain balance. Slowly extend right leg out straight to parallel with floor while retaining upright posture. Hold for 10 seconds, then repeat with other leg. For more support if needed, lean against wall..