When I made the first Squat Rx video about the lower back rounding at the bottom of the squat, as far as I could tell, there were NO videos out there on the topic and it was not a major concern to trainees and coaches. I made it on the fly with a few scratched out notes and threw in a hodge-podge of exercises that I thought might help the issue. It has some rock-solid tips (and some not so solid...), but it's TOO LONG and (because it's too long) it's not as clear as it could be.
Squat Rx #1 - Lower Back Rounding at the Bottom of the Squat
Just last week, Bret Contreras made a new video on the same subject. I like it a lot, but I could not stop laughing at Skelly's silent scream and snapping joints as Bret struggles to make him squat. As with Squat Rx #1, it is too long. Having a tight script, a tripod, and throwing in some ventriloquism would have helped.
Bret Contreras and "Skelly" - Squat Biomechanics and "Butt Wink"
Tony Gentilcore wrote a two-part blog post on the topic. Good but, again, too long.
Fixing the "Tuck Under" When Squatting - Part I
Fixing the "Tuck Under" When Squatting - Part II
A criticism of "Butt Wink" paranoia... Same thing - solid points made but too long.
The Butt Wink by 70's Big
Whew! That's a lot of video and text... DAMMIT! CAN'T ANYONE GIVE SOME SHORT ADVICE ON THE TOPIC?
How To Improve the Bottom Positioning of Your Squat RIGHT NOW
The first two minutes of Squat Rx #1 continues to hold up after all these years. And, distilled into the most basic of basics, I would submit this two-paragraph blog post from 2007 as (still) THE best single tip I can give people wanting to squat deeply better:
If that doesn't work, try some remedial motor control learning tools and cues (such as box squats, goblet squats, "spread the chest", "knees out, etc.), and mobility/stretching drills.