Here's another one:
So he has a slight roll under. He's right on the money about not going deeper because that would just increase it (see more info at right again). Ignore the comments at the bottom.
However this guy would have a hard time maintaining that load at that depth and achieving the lumbar position he wants at the bottom. It that case mobility versus stability is certainly an issue.
But he has his mind in the right place and his butt wink is not SO bad.