Quick version:
A pull, whether short or long, usually means that an offense is targeting a specific area to create a specific hole/seam for the ballcarrier to go through. Let's use a basic fullback trap to the right as an example, against an even front; in this we'll make it a 5-2-2-5, with regular 4-3 defensive alignment.
From the perspective of the offense, the goal is to get the fullback upfield and into a matchup where the first defender he was to worry about is a safety. So, in order from left to right:
- Left tackle ignores the 5 who's just outside of him. The 5 is too far outside to be able to do anything in the backfield, so he can be completely ignored. The LT heads inside and upfield; he can pick up an astute backside OLB if he's there, but more realistically a safety is a target. (In our option offense against a 4-4 type of front, I'd actually have the backside tackle cut the 2 who was inside of him. This would free up our center, but create a huge gamble if the backside tackle missed the cut.)
- Left guard is going to pull down the line and hammer the playside 2. The thinking is that the playside 2 will come upfield, thinking that he has a clear path to the mesh point; that's when the pulling guard hammers him.
- Center will usually down block to the backside 2, preventing him from either ripping into the backfield directly or else latching onto the pulling guard. (Yes, I taught DLs how to hold a pulling guard.)
- Playside guard will make a play toward the MLB. If the playside 2 is extremely disciplined and physical, which means he'll simply squeeze this down block and destroy the play, then we might go at him with an I-block. All that means is that the the playside guard steps backward as if in pass protection, which will bring the playside 2 upfield and allow the trap block to work. My preferred i-block was a drop-step, followed by an outside loop and a trail to the MLB. This has to be done extremely quickly.
- Playside tackle can usually take an inside loop and negate the playside OLB. The 5 who's outside of him is in no position to disrupt the play, so there's no sense in worrying about it.
So on one basic play, the three interior OLs use two down blocks and a short pull. The loop type blocks are more common in an option offense where a defender is meant to be unblocked completely.
Now, look at it from the defensive side of things. The ball is snapped, and:
- The backside 5 squeezes down the line anyway despite the fact that he's unblocked and out of the play. You never know when a guy might try to cut back, or there could be a fumble. Regardless, keep outside contain while squeezing down.
- The playside 5 does the same thing.
- The backside 2 sees the man in front of him (backside guard) vacate and pull; he needs to communicate quickly that there's a pull. While he's doing this, he either grabs onto the pulling guard (which can be flagged if an official sees it) to slow him down, or he can simply follow the pulling guard down the line. But he needs to be wary since that center is on a trail for him at high speed, so he should already have a hand out to fend off the down block.
- The playside 2 should hear the pull call from the backside 2, see the down block in front of him and squeeze it hard, and come flat down the line while looking somewhere ahead of the mesh point.
This is the key part on a fullback trap: the trap block itself. The whole thing is predicated on the playside 2 coming upfield, and the pulling guard's aiming point is the inside shoulder of the playside 2. This isn't a hook-and-seal type of thing; his job is to blast a hole wide open for the fullback to go through.
If he can't do that, the play falls apart. This can be done if the playside 2 slides down the line and slips underneath the pull, or if the backside 2 is able to slip past the center's down block and follow the pulling guard right into the mesh point.
This brief video shows a couple of ways that a playside DT can slip past and create havoc.
https://www.youtube.com/watch?v=I2DvJBHgrGo
Of course, there's no law that says that you can only trap a 2. We used to face 5-3-1-5 fronts, and common sense dictated that we'd trap the 3. Nah...we'd check away from that side and trap the 1 instead, which is extremely intricately timed and is either going to make the 1 regret life or make us regret calling it. Sometimes we'd trap a 3. Sometimes we'd trap nothing (against something like a 4-0-4 front) and have the guard peel upfield, just for a change of pace.
