Offenses and defenses are not static. If one side of the football comes up with a scheme, a route concept or a blitz that is working, the other side is going to come up with an adjustment to stop it, or execute against it.
One of the popular route designs in the NFL today is often termed the yankee concept. At its core, this is a play that uses maximum protection for the quarterback and pairs a deep post route with an intermediate crossing route. The design is simple: Read the safety in the middle of the field and throw off of his decision. If he stays deep to cover the post, throw the crossing route in front of him. However, if he drives down on the crossing route, throw the post over his head.
Defenses have made adjustments. One of the ways teams can still defend this out of single-high coverage schemes is by executing a “cut” or a “nail” call. In this adjustment, the free safety is free to drive downhill on the crossing route in front of him. The cornerback that starts out the play over the crossing route then replaces the safety in the middle of the field.
But the timing is critical.
On this example from Monday Night Football, the Los Angeles Chargers run the yankee concept. They use motion presnap to let rookie quarterback Justin Herbert know the coverage in the secondary. Even though the New Orleans Saints try to make that exchange downfield, Herbert still is able to hit the post route. Make sure the volume is up for this video breakdown: