Originally Posted by
cervicornis
You think I'm lazy because I don't want to turn my head 90 degrees to watch your squat video? Dude, posting a video like that is the very definition of lazy.
But I digress.
Get out of the habit of relying on the mirror to watch or evaluate your form. That is a bad habit.
To me, it looks like your stance might be too wide, you seem to carry the bar a bit too low on your back, and your back isn't really locked out in extension. Have you read the book? Re-read the part regarding back extension and how your spine should be in a normal anatomical arch throughout the movement.