The explanations will be beneath the photos (after the jump).
The play is started. This initial shot of the players' movement on defense tells a lot about their responsibilities. Look at #55 Rivers, the OLB on the left. His feet are planted, he's in total read mode, ready to either retreat into pass coverage or defend the D. 96 Austin heads towards the 'a'; 95 Bernard heads towards the B. 98 Tracy is flowing to keep the C. 94 Kiwanuka was in a down stance for this play and takes on the 'b'. A. Rolle is taking a step outside to fortify the 'c'. That leaves the A gap. Notice the two LBs in the middle: Blackburn, who did not have the best day last Sunday, appears to be spying FB V. Leach. His approach to the line and block could create another gap, which would be unaccounted for. Do you see how the entire gap lineup would shift, according to where Leach approaches? Therefore, the LBs #93 Blackburn and #58 Herzlich have to STOP gap A and the newly created one. Herzlich echoes the stutter step that B. Pierce is taking in the Ravens' backfield. A is green because that seems to be the most likely hole to hit; B is orange because it, too, seems to be developing as a gap to hit.
As the play develops and the handoff is made, confirming it as a run, notice the progression of the gaps being defended. The D is pushed out pretty far, and the C remains in between Ravens Osemele and Pitta. At this point, the B is a legit gap choice, and Bernard is engaged but would have the responsibility if Pierce chose that one. Blackburn still spies Leach and prepares to engage. Herzlich has not fully committed but is dead set on Pierce. Rolle is cheating inside because of the 'b' space and the unseen help he has on the outside for 'c.' The A is clearly the primary aim of this play.
Now, I've changed the colors of the gap labels according to what remains open. The 'c' is well covered by Rolle and the approaching corner from his flank, that you can see at the edge of the shot. The 'b' seems open but is not a shoo-in. The 'a' is locked up because, by design or dysfunction, Ravens C Birk is towards the backfield with his block. Herzlich has closed off the A gap and is about to make contact with the ballcarrier. The B is shut off and covered because Blackburn engaged Leach, pushing his body into the gap. The C is a possibility, but 95 Bernard is taking the right angle and ready for that option, and so is 55 Rivers, back in the picture and attacking that gap. The D is a non-factor. Complete gap coverage.
In conclusion, Herzlich makes a hit, Blackburn actually initiated contact, Rivers closes in, and FS Brown (#27) is coming in to shore things up. This was a 2 yard gain because the Giants covered their gaps well, diagnosed the play accurately, and won some of the 1-on-1 battles. This play is an example of "gap integrity," although Rolle abandoned his gap on the outside as well as Rivers on the other side. That could have been by design.
Now, for a rather glorious example of poor gap integrity by a defense.
Now, look at this slightly different alignment used by the Ravens. The TE Bajema is standing up on this play and slightly split. WR Jones (#12) is in a tight split, causing some gap concern. The Giants are again shifted to the left and assuming the D is outside of the TE.
As the snap is taken, Flacco has a pass look, which effectively freezes the LBs. Look at the stances of the RBs--both have a textbook pause in this moment, to help with the hiding of the play. #93 Blackburn is looking to begin a backpeddle, according to his stance; Bajema fires off the line and is being spied by Rivers. The A gap is opened again. In the in-game announcing, the commentators mentioned that Coach Caldwell, o-coordinator of the Ravens, said that he felt there was something in the middle of the Giants' defense that could be attacked. This is the subsequent play from the abovementioned one. The A gap being this wide is a gamble, which can backfire, as you will see. #58 Herzlich is focused on Pierce again.
The play is revealed as a draw, but look at #93 Blackburn. He was fooled by Joe's quick look to the TE and took steps towards him. This is a weakness in gap integrity for this running play. Do you see why? The gaps have sifhted. #96 Austin is now engaged by LG Jah Reid and fully taking on the 'b' gap. #94 Kiwanuka is also taking this gap. Error #2. I blacked out 'a', because it is nonexistent now, but a dangerous Birk is now free to engage the LB whose responsibility is the back in the gap. World wrecker Vonta Leach fires towards the hole to seal off Blackburn. Oh, and, possibly as design, possibly as compensation for the double-'b' attack, S Rolle charges hard from the right hand side. The B is compromised in a way, yet effectively so because two Giants are there, too.
The C now stands as space but is not really a part of the plan. The D is where 55 Rivers fights his block. Everything else is sealed off and/or accounted for except the A gap. Unfortunately for the Giants, there is NO upfield compensation for the "new" A after Vonta delivers his block. SS Rolle is compromised at the c gap. Who do YOU think is going to win that matchup between 93 Blackburn and Ravens FB Leach?!
Remember, gap integrity has to do with correctly guessing where the D gap will be, and compensating for it, as well as compensating for any reassignment of gaps mandated by a FB. This is why Leach is such a valuable asset, in a league where fullbacks are rare.
By the way, was this play a TD?!? It sure looked like it was from my seat. I also think that if Pierce had simply kept running and not turned to engage the pursuing CB with a stiff arm, he would have made it to the end zone. I'm sure his teammates gave him ample ribbing for that ending. But the beauty of the play stands.