Think of a football field as a flat, two dimensional plane. You attack a defense "horizontally" along a line on this plane. For example, in the All-curl, you are horizontally stretching 4 underneath defenders with 5 receivers all looking back at the QB (versus 3-deep. Versus cover 2 they now have 5 underneath defenders: one for every passing lane). There are 5 passing lanes for only 4 defenders to cover.
This is what would be a called a "short in-out horizontal stretch". The QB is reading inside to out (sit route to curl to flat), on a short horizontal stretch. The key is that you have isolated those 4 underneath defenders in a game they can't win: 4 vs 5.
However, to further facilitate reading these things easily, a coach will integrate a coverage key (here the drop of the middle linebacker) where he will then isolate himself into 1/2 of the field. Then, 5 on 4 becomes the more manageable 3 on 2.
Vertical Stretch:
A good example of a vertical stretch concept is corner/3-vertical route.
First, it is an example of a "deep out-to-in vertical stretch". You want to run this versus 2-deep, so you are stretching 2 deep defenders with 3 deep receivers. The QB would then pick a side based on the safety key, and read outside in (corner to post).
Further, making the play effective is it is also a "hi/lo vertical stretch". On each 1/3 half of the field you can isolate a single sideline defender "hi/lo" with your corner route and your flat--both sideline routes. Essentially this is a 2 on 1.
You do not win football games by creating "one-on-one match ups" unless you have superior talent at each position. You win by getting a numerical advantage, where it is 5 on 4, or 2 on 1.
I prefer 2 on 1s as they are easier--simply look at the movement of one defender--but the practical problems of properly identifying that key defender and being confident no one else will get into the passing lane are not easy, so you go for 3 on 2, 4 vs 3, or 5 on 4.
