With a dumbbell in each hand, bend the knees and hinge at the hips to place your forehead on the top of an incline bench from the back. Keeping the shoulder blades down, squeeze them back together and lift the straight arms out to the sides as high as possible, then return under control. Turning the thumbs down toward the floor will emphasize the posterior delt more and a more neutral grip will pull in more of the lateral delt.
 
Purpose
The head-supported rear delt fly trains the posterior and lateral delts as well as most of the upper back musculature to a lesser degree. Supporting the head on a bench forces a stricter movement, which will keep the focus on the posterior delts, but allows somewhat more cheating than the incline bench rear delt fly.
 
Programming
Generally 10-20 reps are appropriate. Weights will typically be much lighter than expected because of the relative weakness of the posterior delts combined with the necessarily strict movement.