This is a must read. Here's an excerpt:

But the value of all plays comes in their adjustments, and the most common adjustment for the Y-sail play is to tag the play with an "angle." With this adjustment the receiver who normally goes to the flat begins like he is doing just that, but then he reverses field and "angles" back inside on a slant-type route. The reason this works is that the "sail" or "out route" typically pulls a defender upfield; the "angle" receiver runs right underneath him.

This is what BYU hoped would happen when they called the play in the first half. Yet Stoops threw them a twist: he called a six-man zone blitz with an unsound coverage, with only two underneath defenders. In fact, he blitzed a cornerback and a linebacker from the short side of the field — could Hall get the ball off? Keep in mind that while Stoops’s coverage was unsound, it is not irrational. In these situations what he and Brent Venables, his defensive coordinator, like to do is have their defenders play a bit of a "man-to-man" technique. This way the quarterback can’t just throw an immediate pass into a zone void because the defender is playing almost like man.

- Chris Brown, Smart Football