An introduction to protection can be found here.
Regardless of what pass protection scheme an offense is using, they can be faced with more rushers than they can block. When that happens, the offense will need to get rid of the ball quickly (before the untouched rusher gets to the QB), and they will want to throw to the void created by the blitz. Cue the "hot route."
In this example, the offense is faced with a potential of a 7 man pressure against their 6 man protection:
If the defense brings the Sam backer, the offense has him blocked (they have designated him the "Mike"). If either the (real) Mike or the Will come on a blitz the back has them picked up. But if the defense brings all three, the offense has to throw hot. They can do this in a number of ways.
Some offenses and some plays are "built-in hot," while other types of offenses and other types of plays need to be altered on the fly when the blitz comes. Some offenses (typically at lower levels) won't throw hot at all, but will simply keep checking more and more players into the protection.
Built-in hot
Some hot routes are simply built into pass plays, meaning the route doesn't change at all versus a blitz. To illustrate this, I'll highlight a play that Ohio State used frequently under Terrelle Pryor, most notably to beat the Oregon Ducks in the 2010 Rose Bowl. Ohio State packaged Double Slants to the left, with the Snag combo to the right:
Obviously I don't know how Tressel set the protection on this play, but if it were me, I'd make the Will backer the "Mike" (and it looks to me like OSU did).
To the left, you have a very good man route in the slant, which will be very hard for the DB to cover without inside help, and to the right, you have the stick route which is the same way. These quick-hitting routes are perfect for throwing hot, and they allow the protection above to work:
If no one blitzes, the play is run the same way, with Pryor probably going to the snag side and reading the coverage unless he can get the ball to one of the slants because of the way the defense has set. As it played out, Oregon had a pretty complex blitz called. Pryor saw the blitzing DB and immediately went to the slants. A safety replaced the blitzer in coverage, so Pryor kept reading wide and hit the second slant for a first down:
Note that Pryor's read may have been to that side of the field regardless of blitz because of the DBs' alignment. With the CB bailing and the FS the only one who could have covered either route, Pryor can know one of the slants will be open.
Video here. (first play and replay from endzone angle immediately following)
Altering the route
To illustrate this type of hot route, I'll use Arkansas' version of the shallow cross/dig combo, one that Jon Gruden had QB Ryan Mallett break down in this video (at around 2:25 into the video).
The backside wide receiver runs a comeback, while the frontside is on a post. Also to the frontside, the back runs a wheel route and the TE is on a square-in. The slot receiver runs the shallow to complete the concept:
Again with 5 receivers into the route, this is a 5 man protection (Arkansas calls this "scat" protection). The OL has the 4 down linemen and the Mike.
And just like Ohio State's play, if one of the outside backers comes, the QB has to throw "hot". But unlike Ohio State's play, the receivers must also recognize the blitz and convert their routes.
Some teams will have their receivers run an entirely different route if they see blitz, but Petrino and Arkansas run this in a much simpler way. Basically if you are to be one of the "hot" receivers and you see a blitzer pass you to get to the QB, you stop your route short and gear-down where you are.
For the back, this means that if he sees a blitz from his side, he will convert the wheel into a shoot route into the flat.
For the slot receiver, this means that if a blitz comes from his side, he will convert his shallow cross to a sit route or maybe even a pivot.
To this point, our offense has been a long way off of mastering the way that our protections are set. When we do set them correctly, executing is a different problem. But there is a lot that we can get fixed without making personnel moves (say, drafting a right tackle). And because at the root of every successful pass play is successful protection, we need to make strides in this area to have success as a team.
Up next, I'll break down a few plays from the Cleveland Browns' game against the Miami Dolphins and the role that protection played in our successes (and failures).