What actually changed for me, since you asked for specifics rather than theory.
I bought one at roughly your position — squat in the nineties, moving but grinding. The week after, the honest effects:
- Top sets felt tighter, not stronger. No sudden jump in what I could lift. What changed was that the last two reps stopped feeling like my torso was folding slightly before my legs gave out.
- My warm-ups got worse for a fortnight, because I wore it for everything and lost some of the bracing I had built. That was my fault and it is the failure mode the sceptics are describing. I now put it on for the top one or two sets and nothing else, which is the setup I would have started with if anybody had said so.
- Position held better across a set of five than a set of two. The benefit showed up in fatigue, not in maximum effort, which surprised me.
On buying: get a belt of even width all the way round rather than one that tapers narrow at the front. You are pushing into it at the front as much as anywhere, and a tapered one gives you less there — which is exactly backwards for the mechanism above. Beyond that, prong or lever is preference; the lever is faster between sets and a nuisance if you want to change the tightness for different lifts.
And it should be much tighter than a normal belt but you must still be able to take a full breath into your belly. If you cannot, it is too tight and you have removed the thing you bought it for.