To lengthen and relax the muscle at the back of the upper arm, attaching from shoulder to elbow. To stretch the triceps, the Teres, Infraspinatus and Latissimus.
With the arm of the affected side, place the hand at the top of the upper back with the elbow pointing directly upwards toward the ceiling. Place the hand of the other arm at the tip of the elbow of the upwardly pointing elbow.
Slowly and in a controlled manner, gently push downwards on the elbow (towards the opposite shoulder-blade), causing the hand to move down the back. When a stretch is felt, stop and hold the position for 30 seconds whilst breathing slowly and deeply. Return to starting position.
It is possible that at the beginning of your rehabilitation just dangling the affected limb in the starting position is enough to feel a stretch without the need to apply additional pressure.