So if the ball was spotted at the 45, it means this photo is when the refs rule that he began the slide. You can see the ball right on top of the 45.
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At this moment, Shaair has already pushed off the ground (from just inside the 42), and his helmet / arm are even with the 43 as he’s moving forward. He’s already launched his body towards Lawrence, meaning he can’t just hit the air brakes and undo this course of action. There’s also only 6 feet of space between them at this time. If they are both moving 18mph towards each other (which is slow, by peak NFL speed standards) that gap completely closes in about 0.11 seconds. Nowhere close to enough time for Shaair to read the situation and change his course of action, even if he HADN’T already started the tackle.
NFL rulebook may still say that its an illegal hit for anything that happens after the slide starts, even if the defender has already engaged and has no way to adjust. I hope not, but it might. But either way, if you’re a QB, and you’re less than 3 yards away from an oncoming defender when you start sliding, whatever happens to you is on you. The human nervous system and human motor function of the opposing player are not going to be able to react quickly enough to save you, even if they have the absolute best of intentions.