You can find this exercise on page 202 of the book Synergistic Golf.
Exercise Type: BIG BALL
Exercise Category: HIPS, LEGS - FEET, STRENGTH
Stand with feet hip distance apart. Place big ball in center of back between you and wall. Bend knees and squat down to a 90 degree bend of legs. Make sure knees are directly over heels, not toes. Continue to squat 20-30x with control. Variation: Hold squat at bottom with goal being ability to hold 60 seconds.