Reverse Fly

Trains the Posterior Deltoid.

How to do it

Stand holding a dumbbell in each hand and hinge forward from the hips until your torso is close to parallel with the floor.
Let the dumbbells hang below your shoulders with your palms facing each other.
Set your back flat and a small bend at the elbows.
Raise both arms out to the sides until they are level with your body.
Lead with the elbows and keep them travelling outward rather than backward.
Lower under control until the arms hang straight down again.

Pulling the elbows back toward the hips. Back is a row; out to the side is a fly, and the rear deltoid only gets the second one.
Standing up as the set gets hard. The hinge is set before the first repetition; rising turns the movement into an upright raise and lets the traps take it.
Going heavy enough to need a swing. Hinged over with no support, a swing is the only way a heavy dumbbell moves, and a swung dumbbell is not being lifted by the rear of the shoulder.
Rounding the back to hold the hinge. If a flat back will not reach parallel, take a higher torso angle or rest the head on an incline bench — both keep the spine out of it.