If there's one West Coast passing play that has stood the test of time and become a staple in offenses across the country it's the curl-flat route combination. The two routes work very well together to create a horizontal stress on opposing defenses, so well in fact that most every team in the nation from the local high school to Aaron Rodgers' Packers or Tom Brady's Patriots regularly rely on this concept.
The main challenge is defending the curl itself, which when executed properly is very difficult to stop without two defenders. There are different ways the curl route is taught but in essence it's about a split receiver starting fast on a vertical stem but then quickly stopping and turning back to the QB where he finds space. In a standard defense with man coverage or where the corner is responsible for preventing that receiver from getting past them (like cover 3), they have to stay on top of the route, which makes the quick turnaround nearly impossible to stop if the ball is thrown on time.
So the ideal way to defend the play is to have an underneath player drop back into the passing window so the QB can't fit the ball in to the receiver, this is where the flat route comes into play. The purpose of the flat route is to send a receiver running quickly to the flat so that the underneath defender has to vacate the curl route passing window to stop a quick and easy pass from gaining a ton of cheap yards.
If the sam linebacker here doesn't cover the slot (Y) on the flat route he risks that player getting the ball and taking off down the sideline for a massive gain. It's a quick and easy read for the QB and isn't necessarily a difficult pair of throws either, although more accuracy and velocity is certainly more helpful than less.
Of course, defenses are going to do everything they can to avoid getting caught in this simple two man game, which means it's on offenses to find ways to ensure that they can utilize this simple play and go get first downs. Here's how that battle tends to play out these days.
The defensive response
The main way that defenses will look to stop this play is by getting a "three over two" effect on the route combination and ensuring that everyone is in good position to stop the routes from getting open easily. Cover 2 is probably the classic way to ensure that outcome, here's how that looks:
Because the free safety is playing over the top of both receivers, the corner is free to come off the "Z" outside receiver here and defend the flat route, which then frees the linebacker/nickel to get under the curl route and take away that passing window. If either were to make a mistake, the free safety would still be available to help clean up the mess and make the tackle.
There are other ways to defend the play but it generally comes down to the defense getting the safety involved either to free up aggression from other players or to be aggressive himself in attacking the routes. So how do offenses ensure that their QB and skill players can get opportunities to run this concept and enjoy the clear reads and spacing it can provide?
Setting up curl-flat with the run game
Curl-flat is a favorite concept amongst run-centric teams and passing-heavy teams alike because it's such a foolproof way to consistently win when the numbers are even. Spread passing teams hate to get bogged down when they play a team with great athletes that can play man coverage while run-centric teams need to have concepts that are easy to utilize and install but allow them to punish opponents for loading the box.
This would probably be a worst-case scenario for the offense with the defense dropping a safety down to help control the field curl route and outnumber the run but a quick working and strong-armed QB can still hit the throws to the boundary with his boundary receiver running the curl, and in this instance, the fullback running to the flat.
The goal for the offense is to force the defense to play cover 3 or man coverage in order to get defenders in the box to handle their run game and then the advantages of curl-flat can be brought to bear.
Setting up curl-flat with a spot route
The spread offense has led to the rise of teams who don't rely on their run game to dictate terms to the defense but instead do it with their passing game. Instead of occupying that safety by requiring that the defense outnumber the run game to stop it, they're going to occupy an extra defender with another receiver running a route.
One of the more popular ways to do this is with a "spot" route run into the middle of the field from an outside receiver:
Like many spread passing concepts, this one is designed to attack two deep coverage, which as illustrated above is the classic bane of the passing game, and it's designed to do so by attacking the middle linebacker who is often the worst coverage player on the field.
Here the slot receiver runs a spot route, which is like a close range curl route where he's looking for space between the linebackers. If the nickel helps him he can't defend the passing window for the curl route. What's more, it's hard for the offense to help the corner outside covering that "Z" receiver because there are two other vertical threats to the same side of the formation for the safety to account for.
Stopping this play with traditional cover 2 would require that the middle linebacker stop the spot route, which is a bad bet for a lot of defenses. Instead defenses are left to try and involve their safety aggressively to rob the curl or else be very aggressive in using their middle linebacker in coverage, which then makes it harder to present even an honest front against the run and also opens up other vulnerabilities such as this one:
Now if the middle linebacker is pushing aggressively to help to the field in defending the curl-flat/spot combination the backside is left vulnerable to a double slant combination with the slot receiver running into the area left vacant by the middle linebacker as he widens to stop the spot route.
This is a standard way that spread passing teams will use to ensure that defenses are forced to try and defend curl-flat honestly or are punished for choosing otherwise. Either you take your chances with your corner on our receiver or we'll make you live with your linebacker on another receiver.
Setting up curl-flat wIth a shallow cross
This is the Mark Richt method and it's pretty potent because it combines two routes that are very dangerous and hard to stop in the shallow cross and the curl. The shallow cross is a difficult route to stop because it involves a receiver catching the ball on a quick, easy pass at high speed, for that reason it has a tendency to suck in linebackers and open space behind them for the curl since they don't want the shallow cross receiver to catch the ball in enough space to burn them after the catch.
Most offenses tend to use the shallow cross route to throw the ball over the middle on a dig route but for Mark Richt it serves to set up the curl flat combo. Ideally the nickel is sucked in by the shallow route and this opens up the window for the curl with the flat route as a check down.
Setting up curl-flat with with a seam route
For those pesky cover 4 teams that try to get the best of the cover 2 and cover 3 worlds and have aggressive safeties that can make intermediate throws like the curl more risky at times, the preferred way to set up the curl-flat combo is with a seam route by the slot receiver.
For my money this is one of the best ways to attack cover 4 and thus probably the best way in today's game to execute the curl-flat combination. If the nickel turns his back to stop the seam route the curl should be open, if he drops into the curl window the flat route should still be open because the corner can't rely on the safety protecting him from that curl route actually being a vertical and he still has to play over the top of the receiver, making the curl an effective weapon.
Unless the defense rolls over the opposite field safety and leaves the boundary corner totally alone, this is a pretty tough concept to defend. If they do that, they're still left with the boundary corner having to defend a wide range of routes from the backside receiver, even another curl route. This is the way Urban Meyer often sets up curl-flat in his offense and it's a nice way to combine the ball-control nature of the combination with an aggressive play in the seam route.
There's much of the modern game in a nutshell, offenses trying to make things easy for their QBs and defenses doing all they can to stop it. Once again you see the value of a true lockdown corner as the presence of a guy who can make even the curl a low percentage throw without needing help makes it much easier for the rest of the defense to avoid problems. Richard Sherman's ability to do this is a major part of how Seattle gets away with playing curl-flat's favorite target, cover 3, on the vast majority of their snaps.
How does your team set up curl-flat?