A standard passing concept in modern football is known as the switch concept. In essence, it is an adaptable vertical stretch concept that can attack man coverage or any zone coverage.
Teams will adjust to this concept by running a Man-Free of Man-Under defense with an in-out adjustment to the bunched receivers. By playing a hang coverage on the outside, you force the receiver to run a comeback route, which can be driven on and defender. With the safety help over the top, you can essentially bracket the inward breaking route.
Here's how it would look against a standard Cover 3 defense from Miami.
Switch Concept from Trips
Teams often like to run the switch concept from a trips set. This is the same concept as above, it only pushes the switch concept outside and can threaten underneath from the inside receiver (Z-receiver). Often, this inside receiver will run a built in hot route.
Verts Switch Concept from Trips
Now, Nebraska wanted to threaten Miami with a three vertical route concept. Often times, teams will simply run a verts concept: it threatens the seam and over the top quickly. Here, Nebraska slips in a switch concept to rub off the defenders and dictate the coverage a bit (more later). The trips set allows them to threaten screen and overloads one side of the field. But what a lot of teams will do is rotate the coverage to the trips side, and as seen above, the switch concept can be theoretically covered from any sort of Cover 4 or Cover 3 look (by having the FS rotate to the post).
So what Nebraska does here is they move the switch concept inside and run a vertical with the #1. The outside receivers maintains the CBs to their side, while the switch concept in the middle works off the FS. The post goes to the far hash and the outward moving route in the switch concept runs the seam. In this way, it makes it more difficult for the coverage to rotate over to the switch concept and it changes the assignments for the underneath coverage (now a LB will have to carry the seam, or else the back end can't cover it, so over the top rotation doesn't fix the issue).
What the Stack/Bunch Provides
The stack, if the defense doesn’t switch it, almost automatically puts man coverage in a trail position. That allows for the person to be picked on to still be the deep safety. Likewise, often times there will be an “in-out” call that will see one defender with outside leverage and the other with inside leverage. The goal of the receiver running the seam is to get inside leverage on his defender, whether that be a man covering the deep third or someone in man coverage. By starting in a stack set, the offense has dictated that leverage before the snap.
Likewise, the post receiver will look to cross the face of his defender, typically a LB or safety to the inside. This means that the longer route, in which the receiver is looking to beat a man in space to the far hash, is more easy to accomplish. Again, the stack formation dictates this coverage from the defense.
This shows three different ways that Nebraska can utilize the switch concept, a vertical concept that stretches defenses deep. These concepts can be adjusted based on the coverage, but often times a normal switch concept will be seen as more of a 2-high safety beater (nominally), while the inside switch concept will be better against a single-high safety look. In this way, Nebraska saw what defense Miami was typically running, and exploited it.