More upper shoulders / traps stretch than anything. You want full extension through there at the bottom.

But you have to maintain an incredibly tight core and the various shapes you need your back to hit are super important. So you'd never let your spine fully stretch open. That would just cause floppiness that would screw up the timing of the swings and make them less efficient.

In fact, at times I've had a problem where if I just hang and I fully relax my core / back muscles, just the weight of my bottom half feels like it can pull one of my vertebrae in my lower back too far open and I get some shooting nerve pain. So I always keep a tight tension there when hanging.

Maybe light swinging with ankle weights could create more spine stretch. But because of the above, that'd be a huge NO for my back.