This description comes via Ted Seay, via the Chucknduck boards.
Coach: No film, but the following comes from Jody Ashby, on Coach [Andrew] Coverdale's staff at Newburgh Castle HS in Indiana:
Spacing - is the old curl/out route cut in half. So, think miniature post curl, arrow, and a sit route. You can make this look a ton of ways, but I'll give you the most basic way we would run it. Michigan does a lot of this (for your reference). Lets take a bunch formation, the outside receiver who is off the ball runs a miniature curl (4 yards vertical, skinny post to 7, drumroll and stick the toe in the ground, present your numbers to the quarterback, you will get some width on your initial steps to help create spacing), the middle receiver on the ball, runs the mini sit route (he should aim for the far shoulder of the "danger" defender, his depth should be @ 4-5 yards, he is the outlet), the inside receiver is off the ball and runs the arrow route (he should get width fast, aiming point would be 4-5 yards, but should emphasize width first, if he reaches the numbers and hasn't received the ball turn up).
The quarterback uses a big 3, because we want the mini-curl. He will use his shoulders to encourage the flat player to chase the arrow, opening up the mini-curl lane. He will often see the arrow pop open quickly reading flat defenders shoulders, but he will complete it for 2 yard gains, if he's patient, the mini-curl will work itself open. If his mini-curl lane was invaded from inside, he would go to his outlet "the sit." We often have a single on the backside of this and we will always check to see if we can throw single first. If we are unable to throw single, then we should have the numbers (unless 0 coverage) to work the spacing route. If you have "The Bunch Attack" book and look at the mesh route with a stem call, which gives you a sit route (because of the sandbox rule), there you have it - A quick version of mesh stem AKA "spacing."
Diagrams below are mine: