In order for a QB to throw the ball to his right side his feet have to be pointed towards the right. Cornerbacks always look into the back field so they can gauge the dept of a QB's drop back. And as we all know the deeper the drop the longer the pass. So as a linebacker, who's closer to the line of scrimmage, these guys can look at the QB's drop back and which way his feet are pointing at the same time. But the really good linebackers can also gauge how far a QB's feet are spread apart. Which tells the linebacker exactly which guy he's about to throw the ball too.
So a QB has to telegraph his throws before he makes his throws. And a good linebacker can read his body language and make a break on the ball even before the QB throws the ball.
Parsons is a good one.