I had a former trainer walk me through this once. Technically, the difference in a back fly versus a rear-delt fly is your arm position. The rear-delt fly you should have your arms slightly more in a Y shape, to really home in on the delts. If you are doing that move right, you will be able to see yours arms/hands in your peripheral vision. You will feel it in the back of your shoulder. For the back fly, your arms are more perpendicular to your body - more straight out to the side. When you pull back here, you can imagine your shoulder blades wrapping around your spine and you will feel it in your back, not your shoulders. If you just slow them down and do them independent of the video once or twice, really focusing on those mechanics and on where you should be feeling it, you will find the "sweet spot" and then you'll be good to go in the video workouts. If you're not hitting it just right, you'll know it right away and be able to self-correct. After my trainer showed it to me that way, the difference became very apparent and easy to adjust.