This movement is used to isolate the chest and shape the pectorals.
Lie down on an incline bench, with a dumbbell in each hand. Use your thighs to kick the weight up to your chest and then extend your arms over your chest. Start with your arms extended over your chest, as if you have just locked out a dumbbell bench press. Now, rotate your wrists so that your wrists are facing each other. Keep a slight bend in your elbow throughout this movement. Lower the dumbbells out to the side in arc motion until you feel a stretch in your chest muscles. Now, squeeze your chest muscles and lift the dumbbells back to the starting position. Repeat this movement for a set number of repetitions.
This same movement can be done on a flat bench, or a decline bench. Also, there are many machines that can replicate this movement.
Bring the weight down until you feel a stretch in your chest, but do not go too far. This will put more stress on your rotator cuff. Also, make sure to keep a slight bend in your elbow, this will prevent injuries to the biceps tendon.