With a dumbbell in each hand, bend the knees and hinge at the hips to bring the trunk between horizontal and 45 degrees. 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.
 
Purpose
The rear delt fly trains the posterior and lateral delts as well as most of the upper back musculature to a lesser degree. Stricter movement and the arms internally rotated will keep the focus on the posterior delts.
 
Programming
Generally 10-20 reps are appropriate. Weights will typically be much lighter than expected because of the relative weakness of the posterior delts.