Disclaimer: Still early in my disc golf journey (2 years, 860-rated) so don't put too much weight on anything I say.
I'd seen discussion of off-arm movement, "swim motion", "punching down", etc. but had not put any time or attention into my off arm, and figured it was going to need a bunch of work when I got to it. Then I (finally) filmed myself throwing and discovered that my off arm *does* in fact come down nice and tight to my body as I rotate. My conclusion is that the off arm movement is mostly a natural consequence of and counterbalance to the movement of the rest of the body. It should naturally do the right thing - and if it doesn't then you should probably look elsewhere first and fix your pull / rotation / timing / bracing / etc.
Once you've got timing, bracing and rotation perfectly dialed in perhaps you can fine-tune the off arm movement, but until then, don't worry about it. At least that's my plan.
If your off arm is in the correct position during the swing, it will naturally do the swim move.
The issue comes to when you try and force movement with the off arm, such as punching down or punching through over rotating the shoulders.
If you watch guys like simon and paul on their short game off arm, they anchor their off arm Super hard.
Equal and opposite stuff.
If you punch down or through, especially if you do it to early, you're just hurting your power vs assisting the power.
We need to think about additive actions in the swing. Everything is about driving the kinetic chain. If you do any action prior to the chain start, you are arresting the chain and starting over.
If you do actions after, you are driving the chain forward and assisting by actively giving your body something to resist against.
I was doing a bunch of shoveling on snow earlier and thinking about bracing while throwing snow.
It's just like that, you don't throw a shovel of anything off the back leg, you brace and push against the brace to drive the load forward.
It's the same with the arm, if you try and accelerate before, you basically destroy the chain by arresting the chain and starting a new reaction.