We sometimes have a tendency to neglect the rear section of the deltoid muscles. Most figure they will work it when doing back exercises. While this is true, isolation movements greatly reduce the chances of overdeveloped accompanying muscles and compensations.
Stand facing twin pulley cables positioned close together and approximately shoulder height. Grasp cable attachment in each hand. Step back away from machine so arms are extended. Stand with feet staggered. Point elbows outward with arms straight or slightly bent. Pull cable out to sides, maintaining stiff elbow position throughout exercise. Return to original position and repeat.