On our staff, the Y Cross is referred to as being from the (Hal) Mummy Package. From what I have gathered, Mummy used it well, but the play itself has been around since the early eighties and perhaps even before that. I have seen it in Homer Rice's Air Option book and I've seen it in various West Coast Offense playbooks. Sometimes it's called Y Cross and sometimes it's not.
The premise is the weak side flood. X pushes off the deep cover and then either settles underneath an overlapping safety, runs past a hesitating or slow safety, or runs an out around 15 (12 for high school??) against a cover 3 -nobody's-getting-behind-me-safety. The A back free-releases for the flat at 3-5 yards depth looking over his outside shoulder. He should be prepared to stretch the coverage to the sideline. The Y comes across at 10-15 (8-12 for high school?) and looks for an open area behind the weak side linebacker and underneath the safety or safeties. The Z aligns himself slightly tighter to the Y than normal and runs a post reading the middle of field. If it is open he continues the post, if it is covered he can break it off into a dig route if the ball hasn't been delivered. My read vs. a 3 deep would be A to Y to X and vs a 2 deep it would be A to X to Y. (But then again, I like possesion passing on first and second down! The progression on the diagram would be better for a 3rd and 10, I'm sure) The Post can be tagged if determined that it will be open. The sister to this play is the strong side flood, Y Stick. It's reads are almost the same - B to Y with the secondary coverage and personnel dictating whether the Z or the X will be third in the pass progression. Y stick is closer to a 3 step drop than a 5 step drop, however.