Pinching with knees?

I recently switched to a new trainer so I can get some experience on different horses, and I've noticed I've picked up some hidden habits that don't come out when I ride Roxy. The worst of which is probably pinching with my knees. I'm generally fine on the flat, just need a reminder every once in a while, but when as I was going over the pictures and videos from my lesson today I noticed I'm really pinching over the fences. I've never really done this on Roxy (as far as I know), so I'm a bit confused as to why I'm doing it on other horses... Anyway, do you guys have any suggestions on how to prevent myself from pinching? I've been trying to just really open my knees before the jump but is there anything else I can try?
Thanks!

Ride without stirrups!! You can do this on the flat or set up a tiny gymnastic and ride without stirrups over that too if you feel secure enough :)

Haha I do sooo much bareback/stirrupless work already, though I haven't in a couple weeks. But that's a great idea, I'll try it. Thanks!
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Practice more frog position (stirrups ALL the way up, like jockey position).

Hmm, never thought of trying that. Thanks!

What's worked really well for me (and I had to completely retrain my leg because I learned to ride from my knees) was to have my instructor remind me when I got to the fence "Soften your knees!" or "Leg, leg, leg!" I know it sounds kind of silly, but if she can repeat it enough it really does stick.

It also helps if you have a really flat saddle, so you can think about keeping your knees completely off of the leather. If you don't have a really flat saddle see if someone has one you can borrow for a while. The saddle does more than you think it does!

Unfortunately I can't get access to a flatter saddle... But I had my mom remind me while I rode today to soften up and it helped :)