This exercise is used to strengthen your shoulder muscles and also to help strengthen your rotator cuff.
Begin by lying on your side on a flat bench with a dumbbell in one hand. Your other arm can be folded so that you can rest your head on it. While keeping your arm parallel to your torso, create a 90 degree angle with the arm that is holding the dumbbell. Your forearm should now be straight out in front of you, but make sure to keep your upper arm stationary throughout this entire movement. Now begin the movement by lifting the dumbbell up in a semi-circular motion. This is called an external rotation, and you should continue this upward movement as far as you can or perpendicular to the floor. Hold this contraction for a second and then slowly return the weight to the starting position. Repeat until you have completed your set.
Do not allow your arm to drop below parallel to the floor, return to the starting position. It is very important to use a light weight and control the dumbbell while performing this exercise. Momentum may cause injury to the rotator cuff.