I've been having computer issues so this is a little late, but hopefully still somewhat relevant.
Late in the 4th quarter with the Colts driving, Tracy Porter made a play he will remember for the rest of his life. It was a huge play, and all but sealed the Colts' fate and the Saints' first Super Bowl victory.
Pre-snap, the Saints were showing a MOFC look with a single high safety over the top. They were still in position to be able to play a cover-3 as I have diagrammed below, or even possibly something with a cover-2/MOFO shell. From his position in the photograph below, the SS still has a shot to get back to play a deep zone.
Peyton does his fake snap, and the defense shows its hand, shifting a few LBs and bringing the SS all the way up to the line. He has a little time to adjust and think. At the snap, you can see the SS still up close to the line, indicating a single high defense*.
*with a pretty high probability, but nothing pre-snap is ever 100% certain
The Colts sent WR Austin Collie in motion before the snap, passing under WR Reggie Wayne shortly after the snap. You can see Collie passing under Wayne below, while Wayne releases vertically.
In the image above, you can see the Saints' cover-1 man look materializing. Off to Manning's left, you can see the CBs bracketing Collie and Wayne. You can see Peyton reading this half of the field on his three step drop. He probably knew a blitz was coming because the Saints had been sending the blitzes at him all night in passing situations, and they showed blitz before the snap.
Note how there are three Saints defenders who could all be guarding RB Joseph Addai or TE Dallas Clark man to man. What is the other one doing? Blitzing? Faking a blitz and dropping back in to a zone? Could one of the defensive linemen to Peyton's left be dropping back in zone coverage after faking a pass rush?
A lot of the time in passing concepts where two receivers on the same side of the formation both break in toward the middle of the field, you want to hit the outside receiver. This is because the inside receiver (Collie, here), will usually draw any such "robber", "rat", or "robot" (all names for an underneath zone defender), usually leaving the outside man with a winnable matchup.
At this point I am guessing Peyton doesn't think he has to worry about what the extra defender is doing because he knows 1. he is getting rid of the ball as quickly as possible on a 3 step drop, and 2. his receiver is in a matchup he should win every time. So he doesn't care about reading the entire defense, and he really doesn't have the time. We do.
Below, you can see that there are actually no underneath zone defenders, and that the Saints got what they wanted: two pass rushers with only Addai left to block them while three Colts block only two Saints elsewhere. If Manning did not get rid of the ball, he was probably going to take a huge hit.
Also in the image above, you can see Manning "peek" at Clark, who is releasing vertically and running either a normal seam route or some sort of seam/middle read. I am guessing that if Peyton sees this as being more open, he throws it. Instead, he sees the deep safety react toward Clark, and goes back to Wayne.
The play is intelligently designed because of the way CBs typically line up according to a "divider". This is material best saved for another post, but the basic idea is that if an offense wants to create space to pass toward the sidelines, they move the receiver closer to the formation. To create more space over the middle, the offense pushes receivers further wide. Because of this, CBs are often coached to take away that space through their use of leverage. Note the relative positions of the CB to the WR below:
The Colts know this, and probably designed/called this play with that knowledge in mind. Sure enough CB Tracy Porter has outside leverage on WR Reggie Wayne (he was the outside man of the "bracket"). Note how Porter appears to be straddling the bottom of the numbers, while Wayne is somewhere in the top-middle of the numbers
I think that Wayne was supposed to run an "in" here. Watching the video of it, he clearly chops his feet, indicating that he did not intend to make a "speed cut" where he would have just rolled off to the inside with one step. You can see him "gearing down" a bit below:
Wayne may have thought he was supposed to run a "hitch" or a "stop" route where he simply stops and turns around and the ball is there, whereas Manning thought he was supposed to go across the field more as diagrammed above.
Note how at this point if Peyton throws the ball to the point where Collie is now (or a little past where Collie is now), it looks like Wayne should easily make the catch for a first down. Unfortunately for the Colts, it doesn't go down that way. Porter jumps the route, and makes a better break on the ball than Wayne does. Here is a diagram of what I think the called play looked like:
Wayne either slips or runs the wrong route or (in a less likely scenario) Peyton got the route wrong and threw it anyway. My money is on Wayne being at fault, that he simply ran a poor route. Instead of getting inside position on Porter (which should have been automatic for a guy as good as Wayne), Porter is able to jump the route and pick the ball off.
And the rest is history, as they say.