Grab a band with your hands and hold it out in front of you with arms extended and elbows locked. Squeeze your shoulder blades together and pull your arms back until your arms are out at full extension and out at your sides laterally.
Tip: When doing this exercise be sure to keep your traps relaxed and your arms straight so that you do not cheat by using your traps and triceps.
Resistance Band Rear Deltoid Fly Exercise - Start Position
Resistance Band Rear Deltoid Fly Exercise - Finish Position
Resistance Band Rear Deltoid Fly - Shoulder Exercise Video