R and U are behind the QB and can’t see where the grab occurs. It could be face mask, it could be the collar. You can’t guess based on the head twist or you get this:
View: https://youtu.be/kmBgptnhP9E?si=zxnJi7-ecYW4xmSC
LOS officials each have a receiver downfield and are looking down there. Once they “feel” that the QB is about to be sacked they can look back and help on the tackle / get a spot.
In the old days or in college the umpire would be in the defensive backfield and would be staring right down the barrel at this. But the NFL moved the umpire because some guys couldn’t keep up. Once they claimed it was for safety they can’t move him back without the Union asking for truckloads of cash, so there are known holes in coverage.