A lot of discussion over Tua’s knee but...

I was confused as well, I assumed that the ball should be spotted where it was when his knee touched, as on any other play, and in that case it would have been a first down by a long shot.

However the rule is that when a player intentionally gives himself up on a slide, the ball is down from where he is ruled to have begun the process of the the slide.

When you think about it, the rule has to be done this way. If they didn't do it this way, the defense would still have incentive to hit a sliding player to stop his momentum and an offensive player would have incentive to begin the slide but stay off the ground for as long as possible to max out his yardage.

So basically, if a guy begins a slide and is technically off the ground for a few yards before touching, they still spot the ball at the beginning of the slide.

Looks to have started his slide before crossing the marker by this frame: