After the snap
Of course, as a quarterback your thinking and reading a defense does not stop once the play begins. One of the toughest adjustments that quarterbacks have to make is when their post-snap read of a defense does not match-up with their pre-snap expectations.
Take, for example, this interception that Tua Tagovailoa threw against LSU. Nearing the end of the first half, the Crimson Tide face a 1st and 10 in their own territory. The offense lines up with Tagovailoa in the shotgun and he sees this look from the defense:
Now put yourself in the quarterback’s shoes for a moment. Remember Montana’s words. Start with the free-safety. Where is he? Towards the middle of the field. The strong safety? Closer to the box. You see to your left a cornerback in press coverage. What are you thinking? Some variation of a single-high coverage, maybe even Cover 3.
But then, before the snap, what does the defense do?
It is subtle, but the picture starts to change. The free safety starts moving. The cornerback you were looking at on the left starts to bail, while the opposite quarterback rotates down into a press alignment.
The full picture? LSU rotates to a two-high look here, rather than the single high. Tagovailoa reads it partly well, and believes they are in a Cover 2 Man Underneath coverage. However, it is a variation of Cover 2, with zone for the most part, but man coverage over the outside receiver on the right:
Tagovailoa, assuming man coverage underneath, believes that the linebacker will carry the underneath shallow route. Instead, the LB passes off that route and settles into a zone, and he gets right in front of the deeper dig route for the interception.
This is the difficult part of the position in the post-snap phase. With all the variations and adjustments defenses can make, sometimes your own eyes are lying to you, and you get into trouble as a quarterback. Which brings us back to the emphasis of the pre-snap phase of the play. The more information you have, whether due to film study, motion, personnel or a combination thereof, the more informed your eventual decision will be.
Here is another example of a different prospect, from a season ago. Only this time the prospect successfully adjusts his pre-snap expectations, and reads the defense correctly. This comes to us from Daniel Jones against Virginia, while he was still at Duke University. On this play the Blue Devils run Ohio, or go/flat to the left sideline. Jones sees the cornerback playing with off coverage before the play, and as such expects that the CB will stay on top of the vertical route on the outside, and looks at the out route first.
But the cornerback then squats in the flat and on the quick out route. Jones immediately adjusts his expectations, throwing the vertical along the sideline for the score:
The thinking never ends until the whistle. Remember, this is a “huddle to whistle” position.