Start: Grab a very light dumbbell with your nondominant hand and lie on the opposite side with a rolled-up towel on your top hip. Bend your top elbow 90 degrees and rest it on the towel so upper arm is parallel to your torso and your forearm hangs down in front of your abs. Turn your palm toward your midsection.
Finish: Rotate your upper arm backward as far as you can, so the weight arcs upward until it's almost perpendicular to the floor. Keep the same angle in your elbow and wrist, and don't move your shoulder or torso back to get a greater range of motion-it's a light weight, so you have to let your external rotators do all the work. Without pausing, slowly return to the starting position.