You can find this exercise on page 255 of the book Synergistic Golf.
Exercise Type: STRENGTH
Exercise Category: ARMS - HANDS, CLUB CONTROL - ACCURACY, STRENGTH
Get your hands on a tennis ball or stress ball and simply squeeze and release, emphasizing a firm grip. Begin with just a few repetitions in each hand being careful not to overwork the small muscles. Squeeze, release, rotate, re-grip, squeeze again. Switch hands.