So there's a few things there with regards to Rosen.
For starters, he can grip it and rip it to most of the field fairly well.
He arm and release are as good as anyone's if not better.
Rosen also will hang in the pocket and take the hits.
His tendency to do that allows plays to develop....it's also what lead to him taking 5 sacks and 7 hurries in that ugly loss to Texas A&M last year that you pulled some gifs from and 3 sacks from ASU before departing with a season ending shoulder injury.
The week before that ugly ASU game though he played his best game of 2016 vs Arizona and shredded them.
Secondly, I think what you're viewing as "locking on" to receivers is just a delay in his processing information vs certain coverages.
You don't lock on to receivers in only certain areas of the field from my experience but if you don't train your brain to see plays develop vs certain coverages it is possible that he may have a slight delay in waiting for a receiver running a particular route vs a particular coverage.
Instead of throwing the receiver open and anticipating where the spot to throw is going to be, some QBs have to see the receiver open before the throw but it will only happen in certain areas of the field or vs certain coverages.
It's common when you look at QB play but many mistake it for locking on.
Greg Cosell furthers this point here "His 2016 tape showed Rosen at times was a little erratic and inconsistent with his ball placement, but it was not a defining negative. At times he was a beat late seeing, processing and isolating in regard to route concept versus coverage." The link gives a pretty good analysis on Darnold and Rosen by the way and is worth a read.
http://www.latimes.com/sports/nfl/la-sp-darnold-rosen-nfl-20170827-story.html
His mechanics are overall pretty strong.
There are a few things he can clean up there and I think that's really what leads to his biggest knock (at least from me) which is just general game to game inconsistency.
Part of that is because UCLA is just not an elite football program at this point.
Part of that is because Jed Fisch and Jim Mora Jr just haven't solved that riddle yet.
But the mechanics are what lead to his inconstancy with his ball placement.
Overall imo, there's just a lot to like.
There's a big ball of clay that's got a good shape already and you can finish molding him.
Imo, if you trade Tannehill Rosen might be the perfect QB in the mid first and let him sit for a season and let him become the next Rodgers.
That kind of high level upside to be elite is really there with him and it's a question of who can establish the consistency.
I see it much more with him then with some other guys including Allen, Mayfield, and Fitzgerald.
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