Mark Ottobre coaches the single-arm rear delt fly on a cable machine. Grip the cable itself rather than the handle for better leverage, and avoid shoulder rotation at the bottom of the movement to get a full stretch safely.
I feel it all— I don't even know where I'm feeling it. Okay. Here. Okay. When you come over, you're going to be there. So this for me is a full stretch position. If I go any further, I'm going to have to rotate. I don't want any rotation from— and it's out from there. Okay. Also try grabbing it from here rather than from the cable. How does that feel? Better. Good. It makes more sense. And pull back. There. Good. Yes. Perfect. Oh, that hurts.
