The basic gist of Shallow is as follows: there will always be a post route, a dig/hunt route, and a shallow crosser than runs right behind the heels of the defensive linemen at about 2-3 yds depth. From there, you can tag it up in about a thousand different ways in order to accomplish what you want. While doodling/performing thought experiments, I eventually realized that the most important thing is actually limiting how many tags I would want because SO MANY things can work off of this.
The first thing that I wanted to do was establish basic rules for the concept before branching out into tags and these are what I settled on:
1-Tagged receiver runs the Shallow. Typically this is either a slot or a TE, but can be an outside receiver, too.
2-Inside receiver opposite the Shallow runs the Dig/Hunt.
3-Outside receiver on the Shallow side runs a Fade.
4-Outside receiver opposite the Shallow side runs a Post.
5-RB runs a Shoot to the same side as the Shallow. Can be a check-release if you want it.
From there, there's certain tags that I prefer, the foremost being "Drive", where you get the Dig and Shallow coming from the same side, preferably with the Dig being run by the inside receiver like the Levels concept that Peyton Manning rode to his HOF status.
What I like about the Drive concept is that it creates a nice high/low read on the H/C defender while also attacking vertically on the opposite side of the formation.
This one is lifted directly from Chris Brown and Bobby Petrino by extension, I'm just calling it Wheel. Wheel changes the RB's shoot route towards the Shallow into a Wheel in the same direction as the Shallow. The purpose of the Wheel is to clear out a Flat defender who's lurking for the Shallow or punish a Corner who's jumping onto the Post route.
Scissors is a secondary tag off of Drive where your Post and Fade routes perform a quick Switch ala the Run N Shoot concept, but with a hard-set Post and Fade instead of the RNS read-as-you-go. I'd love this for Quarters coverage in particular because I'm doubting the defense's ability to handle the route exchange.
As you can see, Shallow is a terrifically variable and flexible concept and I'm just barely scratching the surface of the play. Hope I presented a good case for it's inclusion in any offense, it's a stone cold lock to be in mine.
Chris Brown's excellent article on Bobby Petrino's Shallow Cross
Brophy's Video Library comes through yet again: Noel Mazzone explains Shallow