You can find this exercise on page 195 of the book Synergistic Golf.
Exercise Type: DYNAMIC STRETCH
Exercise Category: FLEXIBILITY, LEGS - FEET
Stand with feet hip distance apart. Place hands on hips and maintain good posture in upright position with knees slightly unlocked and abdominals engaged. Balance on heels and lift toes up and down as if toe-tapping the sky. Complete a set of 30 reps.