When your shoulders droop or your back rounds at address, you lose power. Want proof? Sit on the edge of a chair and slump forward. Cross your arms over your chest and try to rotate as if you were making a backswing. Tough, huh? Now straighten your back, tilt forward from your hips and try again. See how much more you can turn?
At address, tip your chest toward the ball, pull your belly in and keep your shoulders high. With a straighter back, you'll make a fuller coil as you swing to the top. And that leads to more power.