Strengthens the external rotators (Infraspinatus and Teres Minor). They are much weaker than the internal rotators.
Lie on your side. Your arm is bent to 90 degrees at the elbow and your upper arm is held closely to the side of your chest. Grasp the dumbbell with the thumb pointing up towards your head. The correct position of the dumbbell is shown in above.
Turn your shoulder outward while keeping your elbow bent and your upper arm into your side. See Figure 2. Continue turning outward until you reach the position in figure 3. Then slowly return to the starting position. Then repeat the movement. Notice that the elbow and upper arm remain in contact with the side of the chest throughout the movement. This is very important.