The official tries to determine the receivers intent. Was he running a route = legal. Was he trying to block (screen) the defender = illegal. This is an area where teams are pushing the boundaries of the rules knowing they are not likely to get a penalty. Clemson would argue that their guy was simply running a route and the defender collided with him. If that were true, this is legal. All of what I've said so far assumes the ball is thrown beyond the line of scrimmage. If it is thrown behind the line of scrimmage (that is, the receiver is behind the line) then all bets are off. You can block whoever you want.Can someone differentiate between a legal and illegal pick in football. It's a really crafty move, which almost guarantees an open receiver. Typically you have a pair of receivers that cross paths, and one receiver will look to block or impede the pathway of the defender covering the other receiver. Hunter Renfrow, of course, had a lot of space to catch that 2-yard touchdown because of the screen. Sometimes they call an illegal screen, sometimes they don't. I'm pretty sure the ball has to go pass the line of scrimmage, which it did in the case of Clemson. But do officials also have to judge the intent of the receiver - as in, was the screen intentional or accidental? Anyone care to elaborate? Seems like Notre Dame got flagged for a pick play in 2014 against FSU in a very similar setting.
This play is very similar to pushing a runner. Some may remember the "Bush Push." It is technically illegal to push your teammate who is carrying the ball forward. But you are allowed to block people who are trying to tackle him. So teams push their runner forward and claim they are trying to block opponents. It is illegal - and almost never called. Same thing with the play last night.