You can find this exercise on page 146 of the book Synergistic Golf.
Exercise Type: DB STRENGTH
Exercise Category: CHEST, HEAD - NECK, POWER PRODUCTION, SHOULDERS, STRENGTH
Stand with feet hip distance apart. Hold 3-5-8 lb. dumbbells in hands at shoulders with palms facing each other. Inhale to prepare. Exhale pushing right arm over head and diagonally to left corner. Inhale pulling it back to start and repeat with left arm reaching to the right corner. The same heel of the reaching arm may and should lift off the floor slightly. Repeat 20x alternating punches.