If you are trying to build your rear delts, you need to understand how to hit the muscles in the most efficient way you can.
In the above video, Jeff from Athlean-x shows how to use a single exercise and a single dumbbell to build bigger rear delts without doing reverse flys. The issue with the reverse fly exercise for building your rear delts is that it relies on the use of horizontal abduction rather than the more effective horizontal extension motion.
Think of it this way. In order to get a muscle to grow you want to subject it to the maximum amount of contractile force that you can. A muscle that you can get to it’s maximally shortened position is going to be more effectively stimulated than one that resides solely in the mid range of contraction. If you perform dumbbell reverse flys for your rear delts you are never shortening your posterior deltoid as much as it could be.
To get to this point you need to focus more on getting the elbow behind your torso. This is called transverse extension at the glenohumeral joint. Here’s the even better part about this. On shoulder day (or whenever you are training your shoulders), you can incorporate this direct exercise for your rear delts using a very focused contraction and lighter weights to ensure you do this correctly. Beyond that, however, you can add additional volume by hitting them again on back day or pull day.
You see, the act of transverse extension can be easily incorporated into your back exercises like the lat pulldown, rows, and one arm dumbbell rows. Take your last set of your normal back exercises and lift the elbows and allow them to drift away from your sides to take the focus off of the lats and put it squarely on the rear delts. This additional volume using heavier loads will be a nice compliment to the lighter more direct work that you are doing with the exercise shown in this video.
Speaking of that rear delt exercise, here is the key to performing it correctly. You want to lean against something like the dumbbell rack so that your torso is angled forward without angling too far towards parallel. From here you want to focus on pulling your elbow up and out to the side while simultaneously supinating your forearm. The forearm movement is not meant to engage the bicep but rather to assist in the external rotation of the arm as you raise it up. The posterior deltoid is capable of helping with external rotation of the shoulder and will be maximally activated when you combine the extension with rotation.
Hold the contraction for a brief moment at the top and try not to cramp up. Many who try this movement for the first time will feel an intense muscle cramp because they have never fully been able to get this small muscle to reach such a shortened state. That is ok. Just work on building up your strength in this position and you will see your rear delt growth follow suit.