There's a couple variations in how the shallow is used. Typically, a shallow route is combined with some kind of ten to twelve yard square-in route behind it. The question is from which side does the square-in break from: is he coming from the same side as the shallow -- i.e. trailing him? Or is he crossing over the top? This matters because it makes a big difference between what kind of flow you're looking for from the linebackers and where the windows in the zone will be. The Airraid guys tend to have the two guys cross and head in opposite directions. the diagram below.
(Image courtesy of Bruce Eien; check out more notes on their version here.) For clips of this version of the concept try here and here. It's a great concept: the shallow pulls the linebackers while the square-in -- aptly known in Airraid terminology as a "hunt route" because the receiver "hunts himself a good place to settle down" -- find the void between defenders.
I am concerned in this post, however, with an older version of the shallow. This is the "drive" concept, made popular by Bill Walsh as a way to get his flanker or "Z" receiver (Jerry Rice, hello!) the ball running full speed across the formation. Secondarily, if they covered Rice there was usually a big window in which to throw the ball to the tight-end. Below is a diagram of how Walsh ran the play from several different sets, straight from an old 49ers playbook.
First, a few features of Walsh's version. Keep in mind a very basic fact that many NFL announcers do not understand: The slant and the shallow are not the same play. On the slant, the receiver takes between two and four steps upfield (I prefer three, beginning with the outside foot back) and then a 45 degree break. The quarterback takes a three-step drop and delivers it quickly. The shallow, on the other hand, takes awhile to develop. The quarterback takes a full five-step drop (and sometimes seven, or five from shotgun) and waits for the receiver to at least cross the centerline. Again, it was Walsh that helped systematize these plays, though they preexisted him.
Anyway, his "drive" route, as he called it, begins with the shallow. Walsh often motioned his flanker towards the formation to enable him to explode through all that man-traffic so he would hopefully emerge free and clear on the backside. If the defense was in was man-to-man -- especially loose man-to-man -- the cornerback usually had a hell of a time trying to catch up with the receiver. But if the defense did cover that route, the tight-end was responsible for finding the void in the zone. The final read was the runningback as an outlet.
But, before getting to those routes, the quarterback always retained the option to throw deep to the corner route or the "go" route on the backside; typically this was the case if the defense was in man to man and he saw a matchup he liked. Note how Walsh sometimes had the corner route run by the halfback in the backfield -- think of Roger Craig or Ricky Watters with a linebacker assigned to him. Alternatively, sometimes the cornerback and near safety were so concerned with the flanker and tight-end that they collapsed down, leaving the runningback or slot man to break wide open on the corner. Nowadays, many modern teams send the runningback on a "wheel" route -- i.e. begin the route like you are running to the flat on a "shoot" and then turn and "wheel" it up the sideline. Below is a variant Bobby Petrino has used in the past at Louisville, the Pros, and now Arkansas.
So where are we now with the play? There have been a few changes in how most teams use the route and concept, especially since now that so few teams base out of two-back sets. As a result, the shallow is seen more as a route for inside receivers than a way to throw it short to your flanker. Below is a diagram of one of the modern versions (as you can see, quite similar), explicated in rich detail in Dan Gonzalez's book, Concept Passing: Teaching the Modern Passing Game. Gonzalez calls his route concept "drag."
So the basic route structure is the same as Walsh's drive. The major differences are that interior players are used, that the outside receiver to the trips side is given a post route to exploit a safety who might play the square-in too aggressively, and on the backside the X is a greater part of the play.
The quarterback isn't always told to read the comeback, but if he does he keys the outside linebacker to that side. See the diagram below, the man circled in blue. If the linebacker drifts where he is or takes the shallow coming to him, the quarterback can drill the ball to the comeback on the outside. If the linebacker heads quickly to the flat to take away the comeback, the shallow route should come wide open in the void he has vacated.
The rest of the play works just like Walsh's drive: either the shallow or the square-in should come open depending on who the linebackers pay attention to, and if neither is open the runningback should be open. Below is footage of the play, thanks to Dan Gonzalez and Mav (first clip), and Bruce Eien (second clip). Bruce's differs slightly but is similar enough to be grouped into the same family.
The film by Eien, directly above, is basically a cross between the true, old school "drive" and the Mark Richt/Florida State version, which again is the same "drive" concept but applied to a more two-by-two setting. For more on Richt's version, check out Trojan Football Analysis. Below is a diagram and cutups of the concept as another comparison.
As evidenced by my old post on how Mike Martz used the route, the shallow's usage is limited only by the imagination. It can be the primary route, the hot route, or a secondary route for the quarterback to come to late in his progression. It can be run by any receiver, and can be part of a main concept or used in some auxilliary way. But however it is run, it is still one of the best ways to get the ball to a playmaker -- speed in space -- and so long as teams keep spreading their formations, they will keep using the shallow cross.
Bonus - Video below of Mike White, then head coach of Illinois, explaining the now-Airraid version of the shallow cross. His quarterback at the time was Jack Trudeau. After diagramming the play White shows the square-in or "hunt" going for a big touchdown against one of Bo Schembechler's Michigan teams.
[Ed. Note: Extra thanks to the guys at the Huey Board for some of the diagrams and info, in particular Mav.]