Lie on a flat bench (or on the floor) and hold a dumbbell in one hand, with your arm straight up over your shoulder and your palm facing away from you.
Bend your elbow and slowly drop the weight toward your opposite shoulder. When the head of the dumbbell touches your shoulder, reverse the move and return to the start.
Lightly place your non-working hand on the outside of your working elbow to steady and support it, helping maintain proper form.