Many defenses pride themselves on being able to get pressure on the quarterback with just four down linemen. This helps the the rest of the defense because if you don't have to blitz to get pressure, then you can drop seven guys into coverage.
If your offensive line is having a hard time keeping the quarterback clean, a good halfback screen can help to take advantage of that pass rush and even slow it down if you run it successfully enough times.
Everyone has an important role to play on a screen pass, and several of the players have to be good actors. There is only one receiving option on the play, and that's the tailback.
The reason for this is if the running back falls down or is covered, then the timing of the play is thrown off.
The offensive linemen are taught to get down the field to block for the tailback, so if the quarterback tries to complete a pass to someone else, there is a high likelihood the offense will draw an illegal-man-downfield penalty.
The receivers have to accept that while they won't have a shot at catching a pass, they still have to run good routes to draw defenders down the field. If they don't, then the screen won't have much of a shot of being successful.
The green wide receiver to the side of the screen has to try his best to get the corner to follow him deep down the field. He should run outside at the snap and run a great go route as if the play was drawn up for him.
The yellow slot receiver should get up the field for a skinny post, but at the top of the route, he should be looking for a safety to block. The backside brown and blue receivers just want to try to keep their defenders backside, so they run a short corner route and go route respectively.
The offensive tackle and offensive guard to the side of the screen have to pass-set real wide and invite the defensive linemen inside, almost as if they are zone run-blocking. This allows the quarterback to really sell the screen without someone getting right in his face or getting in the passing lane between himself and the tailback.
The center needs to also emulate zone blocking towards the side of the screen. The backside offensive guard and offensive tackle have to set inside hard to prevent an inside move from the defensive linemen in front of them. Then, as best they can, they should try to hinge back out and force the defensive linemen out wide.
The tailback has to take a track as if he will be getting the handoff for a zone running play. This action should cause the linebackers to take a step up into the line. When they see the quarterback fake the handoff and continue to drop back to pass, what you hope is that they are most concerned with recovering from their false steps towards the line of scrimmage, forcing them to turn and run to get to their coverage responsibilities.
This would preclude them from noticing the tailback hanging out behind the offensive line, waiting to turn around and catch the screen pass.
The quarterback has to put on an Oscar-winning performance and be fearless because more than likely he is going to get hit on this play.
After faking the handoff, he continues into his regular pass drop. He has to pause just for a second and then continue backward as if he is just reacting to the defensive linemen coming to sack him. He can't be too obvious, however, because some defensive linemen are smart enough to know they are being set up and will run back to try to tackle the running back or just stop and try to jump and knock the pass down.
After getting the linemen to continue their chase, the quarterback has to throw an accurate pass high enough to sail over their heads but hard enough to get to the running back quickly; he can then turn around and run at the earliest possible moment.
One of the most important coaching points for the offensive linemen who will be leading the way for the tailback is that they can't be concerned with what is going on with the quarterback and tailback. They have to trust that those guys will do their job, because if they turn around to see what's happening, they will lose sight of the defenders they have to block downfield.
It's already going to be hard enough as a big guy trying to spring 10 yards down the field and block a little corner or safety that is trying to put a juke move on them; there's no reason to make it harder by losing track of where they are.
If all else fails, an athletic quarterback can try to break to the outside and throw the ball away.
All in all, this is a low-risk, very high-reward play that cannot only positively affect field position but also affect the opposing team's pass-rush plan.