Originally Posted by
Satch12879
First, "buttwink" is not the thing you are worried about, if it is a thing at all, which it's not. You exhibit lumbar flexion, on top of other form errors that Jeff identified. "High volume" squats done with a squishy low back is likely the cause of your low back soreness, yes.
Keeping tight is all well and good, but you need to learn how to control your thoracic and lumbar spines. It is a skill that you have to learn away from the bar if necessary. Keeping tight is just one part of the equation.
The lack of thoracic extension is indicated by the fact that your upper back is rounded and your chest is not up.
Jeff recommended bringing your heels in because too wide of a stance will result in things reaching the ends of their extensibility preventing you from getting depth. Thus, if you force through that, in order for you to get depth, something has to relax, and usually it ends up being your low back as we can see.
Regarding the knee slide, on top of what Jeff recommended, a cue that worked for me is to think about tucking your crotch down and back under your butt as you descend. When you do this correctly, you will feel your hamstrings stretch. It has the dual benefit of helping you keep your lumbar arched and preventing you from humping forward at the bottom.
What are your work set weights and what does your programming look like?