Reverse Machine Fly

Trains the Posterior Deltoid.

How to do it

Set the machine's arms forward so you start with your hands together in front of you.
Set the seat so the handles sit level with your shoulders.
Sit with your chest against the pad and take a handle in each hand, palms facing each other or down.
Set a small bend at the elbow and hold it.
Draw both arms out and back in a wide arc until they are level with your body.
Return under control until the hands are back in front of you.

Squeezing the shoulder blades together to move the handles. That is a row with straight arms — the rhomboids and middle trapezius do it, and the rear of the shoulder gets very little. The blades stay still while the arms travel.
Setting the seat low so the arms travel upward and back. The path is horizontal at shoulder height; angling it upward brings the traps in.
Going past the point where the arms are level with the body. Further back is the front of the shoulder being stretched under load, not the rear of it working.
Loading it heavily enough to need a shrug. The rear deltoid is small and works through a long lever here, so the weight that works is much lighter than the machine's stack suggests.