This was me in October of 07 after lifting seriously for about 6 months or so. I bought into the blind ATG Nazi cult that seemed to hold sway over some forums at the time, and this was how I squatted.
Gah, I want to delete this video and forget that it ever happened but I figured that some people might benefit from seeing it so it remains on my youtube account.
This is as they say “buttwink” which is the lower back rounding when hitting depth in the squat. This is akin to rounding in the deadlift, which is obviously something that should be avoided at all costs.
Hip mobility is the answer.
I’m a little older and a little smarter now, and I just hit 3 plates in the low bar back squat a month or so ago.
Bottom line is, squat as low as you can without rounding and stay there and slowly increase your mobility until you can squat properly.