First, anyone interested in a great discussion about understanding passes and how they affect defensive structure should check out this
thread from the Chuck n Duck forums.
I want to give lots of credit to Coach Bill Mountjoy who posts on there, as he provided most of the most useful information:
...[Mike] Martz & [Joe] Gibbs are disciples of the [Don] Coryell offense.
You have Horizontal Stretches (Inside/Out, AND Outside/IN) with either 2 on 1, or 3 on 2 (USUALLY in 1/2 of the field - deep OR under).
You have Vertical Stretches with 3 on 2, or 2 on 1 (USUALLY in 1/3 of the field).
You have "Objective Receiver Concepts" (which is with ANY pass in which a specific receiver is primary - such as "OPTION ROUTES", ETC.).
You have read concepts (below) to facilitate the above: NOTE: "MOFO" = MOF OPEN; "MOFC" = MOF CLOSED).
QUARTERBACK READ SYSTEM:
KNOW THE SITUATION
PRE-SNAP LOOK THE DEFENSE – ANTICIPATE
READ ON THE DROP – ADJUST
THINK: PROTECTION/ADJUSTMENTS/PROGRESSION/TIMING/OUTLETS
BEWARE OF THE MIDDLE OF THE FIELD LOOKS – MOFO/MOFC
KNOW THE COVERAGE ELEMENTS:
ZONE 3 DEEP MOFC/2 DEEP MOFO, etc
That's a handful. Quick notes:
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). Technically some of these receivers are at 3-5 yards and others are at 10, but it constitutes 5 passing lanes for only 4 defenders to cover.
This is what would be a called a "short [or intermediate] 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.
Further, a great play is the
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). Again, if you can isolate the defenders at this level, it becomes the classic game they can't win: 2 covering 3.
Further, making the play effective is it is also a "hi/lo vertical stretch". In this case you hopefully, on each 1/3 half of the field, can isolate a single sideline defender (the squat-cornerback versus cover 2) who you can attack both high and low, or "hi/lo" with your corner route and your flat--both sideline routes. Essentially this is a 2 on 1.
The point here? You do not win football games and complete passes by creating "one-on-one matchups" unless you have superior talent at each position. You win them by getting a numerical advantage, where it is 5 on 4, or 2 on 1.
We prefer 2 on 1s and 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.
This is intended for zones, but how do you attack man? I will save some of these ideas for another article, but suffice to say that many of the best coaches will:
1. Have the individual routes that attack the zone be effective versus man (corner routes, shallow crosses)
2. Integrate certain anti-man concepts within a zone stretching framework (such as the mesh or option routes)
3. Put man combinations to one side and zone combinations to the other. Many of the best NFL and College teams do this quite effectively, and it is still simple to do.
ConclusionMany of you probably have questions about how a Quarterback actually goes about deciding who to throw to. Even if he has a 5 on 4 situation, how in the world does he quickly determine who to throw to? Coach Mountjoy provides an excellent and quick rundown below:
You can read defenders OR progressions. Examples below:
DISCUSSION OF PROGRESSION READS AND COVERAGE READS
I. PROGRESSION READS: A progression read is designed to have two or three choices of where to go with the ball. It is important to pre-read the coverage to give you an indication of the coverage, but more importantly, it’s knowing where the receivers are going to be with a progression read pattern called. This kind of read calls for throwing the ball with rhythm drops. You might get to the third receiver in the progression as soon as you hit your fifth step on the drop. So when you are stepping forward to throw, you can hit the third receiver in the progression on the same rhythm you would have if you were throwing to the first.
The limitations of progression reads are:
A) There is a tendency to stare at the receiver that is first in the progression attracting other defenders
B) It is frustrating for coaches to watch because they could see the receiver you didn’t throw to was wide open (Coaches need to know the progression of the play as well as the QB); [i.e. QB threw it to the first read who was kinda/maybe open and #3 was uncovered].
C) You will lose patience or think that because you hit the first receiver in the progression he won’t be there when the play is called again. You must have patience and not make up your mind before the ball is snapped.
REMINDERS:
1. Have a plan when you get to the Line of Scrimmage.
2. Stay with the progression.
3. Don’t stare.
4. Progression reads are thrown with rhythm drops.
II. COVERAGE READS: Reading the coverage is normally done in the NFL looking at the pictures that are taken upstairs during the series (when the QB is on the sidelines). In High School & College – the Press Box Coaches do most of the work here. The QB can pre-snap read and get an idea of what might happen. He can see rotations and drops of defenders at the snap of the ball, but may not know what the coverage was. Reading the coverage is really looking at a defender or defenders. Based on what they do you will get to the correct receiver.
THE ADVANTAGES OF THIS KIND OF A READ ARE:
1. It eliminates the struggle of the progression read trying to determine who was more wide open.
2. It eliminates the QB from making up his mind before the snap (we shouldn’t do this regardless of if we Progression Read OR Read the Coverage). Read the defenders to get you to the right receiver in Coverage Reads.
3. It keeps the QB on the same page as the Coach because they both know the read and the goal of the play called.
4. It doesn’t matter what the coverage is because when you are reading properly you will be hitting the correct receiver.
5. You will not have to stare at your receivers (it will give you natural look offs).
6. You don’t have to know what the entire coverage is (you don’t have to see the whole field). NOTE: In our reads – “Progression” AND “Coverage” – we only read ½ the field Horizontally, or 1/3 of the field Vertically.
