You can find this exercise on page 43 of the book Synergistic Golf.
Exercise Type: DB STRENGTH
Exercise Category: BACK, POWER PRODUCTION, RHYTHM - TIMING, SHOULDERS, STRENGTH
Stand with feet slightly wider than hips. Unlock knees and hinge from waist so back is flat and parallel with floor. Arms are hanging straight down from chest holding 3-5-8lb dumbbells facing each other. Inhale pulling one arm up as other reaches farther to floor then exhale switch with slight rotation of torso with each pull and reach. Focus on control and abdominal support for back. Repeat 20x alternating pulls.