The reverse cable fly is an isolation exercise for the back head of your shoulder muscle, the posterior deltoid. It will also work your traps and upper back as secondary muscles, and your core to stabilize the movement.
The shoulder muscle consists of three heads. Most people tend to develop their front and middle delts with chest and shoulder presses while neglecting the rear head. This limits your shoulder size and strength. Even worse, weak rear delts cause strength imbalances in the shoulder. Overtime, this imbalance causing the shoulders to round up towards your front, giving you bad posture, and greater risk of shoulder injury.
The reverse fly can be done with cable or dumbbells, like with the bent-over rear delt raise. One major benefit of the cable is that it provides a steady resistance throughout the full range of motion. While with dumbbells, due to gravity, most of the pressure happens near the end of the movement, when the arms are extended to the sides.
When getting into position, move a step or two back so that the weights will raise a little off the stack. This way, the tension will stay on your delts the whole time, allowing you no resting point.
Keep your torso upright and still. Do not rock it back and forth while pulling, this will remove the pressure off your delts to the back muscles, making the exercise less effective.
Keep your arms straight, but allow a slight bent at your elbow, to protect the elbow joint.
Don't lift your arms over the shoulders. To target the posterior delts, your arms should move directly back and slightly downwards, almost parallel to the floor. If the hands are raised over your shoulder this will distribute some of the pressure to your lateral delts and traps.
Move your arms in steady and controlled pace. it is easy to use momentum with this exercise, but it will make it less effective. Remember that the negative part of this exercise is just as important as the concentric part. So don't just drop the handles fast, but move them slowly to the starting position.