How to Perform the Cable Reverse Fly
The deltoid muscle group is made up of three heads: anterior, lateral, and posterior. Many athletes emphasize exercises targeting anterior or front deltoid-dominant which can lead to overdevelop front delts and underdeveloped lateral and posterior delts. As a result, they have an unbalanced physique, poor posture, limited range of motion, and an increased risk of muscle impingement.
The standing cable reverse fly is a great isolation pull exercise used to target the posterior or rear deltoid muscle. The lateral or side deltoid, middle and lower trapezius, rhomboids (middle back), infraspinatus and teres minor (rotator cuff) act as supporting muscle groups during the exercise. 
Supporting muscle groups assist the target muscle group(s) during the movement. The triceps brachii, wrist extensors (outer forearm), and erector spinae (the long muscle running along either side of the spine) act as stabilizers during the movement. 
Stabilizer muscles help maintain a posture or fixate a joint by contracting without significantly moving.  Shoulders can be hit with a higher frequency and volume compared to other muscle groups because they're comprised of a higher percentage of slow twitch, endurance muscle fibers.
How to Perform the Cable Reverse Fly
Begin by approaching the cable tower, moving the pin heights on both sides of the tower to shoulder height, attaching the single handle pulley attached to each side, and selecting the appropriate working weight. Although you can perform reverse flyes one arm at a time, this walkthrough will cover performing the exercise standing and use both arms at the same time.
don't use 10lbs on one side and 20lbs on the other side. Uneven loading won't improve your gains and will likely lead to an injury. If this is your first time performing the exercise then pick a conservative weight that you can safely lift for 8 to 12 repetitions.
Once you've selected the appropriate working weight, grasp the handles with a neutral/hammer grip (palms facing each other). Ensure your right hand is holding the handle from the left side of the tower and the left hand is holding the handle from the right side of the tower. This will maximize range-of-motion.
If the cable tower is relatively wide then you can walk over to one side, pick up the handle, and then walk to the other side and pick up the other handle. Before beginning the exercise you should be standing in the center of the cable tower so that the tension on each cable is about the same. For many cable towers, this means you're standing directly under the pull-up bar.
Take a stance in between hip and shoulder width; your feet can be staggered or in-line with each other. You can use a traditional grip (thumb wrapped around the fingers), hook grip (fingers wrapped around the thumb), or a false grip (thumb and fingers on the same side of the handle).
After setting your stance and grip, take a deep breath, brace your abdominals for impact and stand tall with an upright posture and chest high. With the handles starting at shoulder height and your elbows straight (but not hyperextended) or slightly bent, begin pulling the two handles apart and away from each other.
Maintain the same elbow position and continue pulling both handles apart until your upper back muscles are squeezing together and your elbows are in-line with your shoulders and ears. Hold the cable handles at this position for 1 to 5 seconds. Then in a controlled motion slowly allow the cable handles to return back to the starting position. Complete for the desired number of repetitions.
Some lifters choose to exhale while pulling the handles apart, at the top of each rep, or in between in each repetition. Choose a breathing pattern that feels the most natural and comfortable for you.
Throughout the entire lift, your chest should stay high, your arms should remain in-line with your shoulders, and your elbow position should remain fixed. If you find yourself swinging your body, raising your shoulders towards your ears, or unable to control the cables as they return back to the starting position then the weight is too heavy.
This exercise can be performed using straight sets, pre-exhaust sets, drop sets, rest-pause sets, supersets, trisets, giant sets, paused reps, partial reps, forced reps, or slow negatives. If your posterior delts are seriously lagging behind in both size and strength, then prioritize them by placing them first on your shoulder, back, or pull day(s).
As with any exercise, the two most important components are high-quality form and progression. Progression can take a variety of forms (e.g. more weight, sets, or reps, decreased rest period, improved rep quality, etc) but strive to improve every training session.
Cable Reverse Fly Form TipsHold at the Top - If you're looking to increase intensity then experiment with holding the cable handles at the fully pulled apart position for 5 to 10 seconds. Really focus on squeezing the shoulder blades together; imagine some placed a pencil in between them and you're trying to pinch and snap it with your upper back muscles.
This will increase time under tension and the burn in the posterior delts. Increased time under tension is an excellent variable to adjust for progressive overload and enhanced muscle growth.
Take a Pronated Grip - Instead of taking a hammer grip (palms facing each other) try performing the reverse fly with a pronated grip (palms facing the ground and knuckles facing towards the ceiling). You may experience an increased range-of-motion and contraction of the posterior deltoid muscles due to the slightly altered movement pattern.
Use One Arm - If you're having trouble getting a good posterior deltoid contraction or if one posterior deltoid muscle is lagging in size or strength compared to the other, perform the cable lateral raise with one arm at a time. This will help improve the mind-muscle connection during the exercise as well as even out any imbalance that may have developed between the rear delts.
To perform a one-arm variation simply take one hand and hold the tower or place it on your thighs or hips and raise the cable using the other hand. Perform the desired number of reps and then switch hands.
Avoid Momentum - The reverse fly provides maximum benefit when it's performed in a slow and controlled full range-of-motion. Check your ego at the door and don't attempt to reverse fly 100lbs.
Excessively heavyweights on this exercise will lead to body swinging in an effort to generate enough momentum to raise the bar. This momentum dramatically increases the likelihood of injury and minimizes the stimulus of the target muscles.
References1) "Cable Reverse Fly." ExRx (Exercise Prescription) on the Internet. N.p., n.d. Web. 29 Oct. 2015.
2) "Kinesiology Glossary." ExRx (Exercise Prescription) on the Internet. N.p., n.d. Web. 29 Oct. 2015.