You can find this exercise on page 295 of the book Synergistic Golf.
Exercise Type: DB STRENGTH
Exercise Category: LOW BACK, POWER PRODUCTION, RHYTHM - TIMING, STRENGTH
Stand in address position with 10-15-20lb dumbbell held in both hands against sternum. Head looking at ball. Rotate only trunk back and forth quickly in short arc. Focus on stabilizing with legs and gluteals while trying to increase shoulder turn. Repeat 50x. Breathe and maintain good posture with activated abdominals.