I’ve followed your pointers. Narrowed my stance slightly (my groin feels much better for this, not getting that lock) and the prompt regarding the knees. I’m not feeling my glutes coming into play which is definitely good.
The Butt wink is still there though. Any other tips?
Ho-kay.
I think there's a core bracing component (not enough abz/obliques) and you're also hitting depth by drifting forward (notice how your ankles start moving as you're just above depth)... Part of the problem - I think, watching your t-shirt move - is that your core bracing isn't actually stopping your lumbar move (it moves at the top - indepedently of your thorax - and, unsurprisingly, at the bottom, too). You have to squat as if your core (hips through to shoulders) are rigid, so sitting back SHOULDN'T happen independently of your thorax tilting.
When you brace your core, it should feel like you're trying to rid yourself of the ultimate battler: everything should be as tight as you can make it in the right position. My old coach got me to test this by coughing whilst pinching in an inch from the iliac crest: if that is not rock hard, you're not doing it right. I've just had a go myself (grown a gut specifically for trying low bar) and you need to get that squeeze on tight.
There are some genuinely unpleasant stretches for your hip capsule area (couch stretch, pigeon stretch and frog stretch - I don't make these names up) that will probably have you screaming in no time. Once you've done your warm up sets, give these a go for a minute each (and each side) and then carry on with another warm-up set before your working ones. If there is any more hip mobility to get, you'll find it here.
Also, what is this about no glutes? That is not good.
Pull your knees out wide to keep them busy. This will also help you create that additional space for your trunk, too.
