The “Yankee” route concept is a 2-receiver route combination: a post and a deep crosser coming from opposite sides of the field. It’s a shot play, predominantly called on early downs when the defense is anticipating a run. As a result, it’s often called out of bigger personnel groupings and paired with play-action.
This first example from a game between the Ravens and Bengals during the 2023 season gives a pretty clear-cut view of the play. Baltimore came out in “21” personnel and tight splits on 1st down. You can see the route combo illustrated below:
The personnel, formation, and down (combined with the fact that the Ravens were a high-frequency running team) got the Bengals into a Cover-3 look:
This is an ideal coverage to run this concept against. The quarterback reads the deep safety. If he bites on the over-route, throw the post. If he stays deep to take away the post, throw the over.
Here, the safety bit on the over-route: