2 Gap Responsibility:
Defensive linemen have to monitor 2 different "gaps" or "zones" along the offensive line. This is each gap on either side of a designated offensive lineman across from them. Once the ball is in play the defensive linemen must read their "keys" to see if the play is a run or a pass and to which side of the field the play is designed to go. All of these keys dictate which gap they try to maintain against an offensive linemen.
1 Gap Responsibility:
Defensive linemen are instructed to rush through a specified zone in passing downs or in an obvious running down maintain the one gap integrity. In the possible running scenario the linemen look to see if there is a run to begin with. If there is a run they maintain their gap discipline. If the play is actually developing into a pass they then stay in their gap but try to get to the quarterback.
The Defensive Line During a Running Down:
In a 2 gap scheme the defensive linemen have more onus on "covering" or staying on the block of their offensive linemen. The point is so that the inside linebackers (in this case, a 34 scheme) can make more plays in coverage or in the running lanes. In a 1 gap scheme the inside may be adversely affected by free guards who may press forward during running play and thus block them out of the play. This is often called by the announcers as the offensive linemen "getting to the second level of the defense."
The Defensive Line During a Throwing Down:
The 2 gap, specifically for defensive linemen, is very much a behind the scenes/low credit given type of role. Their responsibility to cover their offensive line counterparts (key word is that it's plural not singular) often leaves them to do the dirty work of blocking while the edge outside linebackers rush the quarterback. Yes, there are the few who can amass four to eight sacks a season from the defensive end position like the recently retired 49er Justin Smith but again, the defensive linemen are asked to block the offensive linemen long enough for their counterparts in the linebacking core and possibly the secondary to get to the quarterback.
For the 1 gap scheme it's really more about pass rushing moves and who has the best moves at the right time. Imagine a starting gun at a track meet. Once the quarterback goes into his cadence and shouts "hike" any and all the linemen have to maintain their one specific gap responsibility but also try and interfere in anyway necessary with the quarterback and his potential attempt to throw against their defensive unit.
Example of Good Gap Responsibility but Bad Tackling by Second Level
From Bottom to Top and their Assignments:
Chickillo has to take up both the right guard and tackle so that the outside and inside linebackers can have a clear viewing lane to the backlfield. Huertelou plays as the nose tackle and his job is to cover the center and possibly the left guard if he blocks down. Pierre is assigned the left guard and tackle so that Burns can see into the backfield. Lastly, McCord is to seal the edge where the tightend is crouched.
SEQUENCE BREAKDOWN 1
The right guard is pulling away from Chickillo as he's engaged with the right tackle. Both Pierre and Huertelou are caving their assignments in to the pocket which is an above average play, they aren't giving ground. McCord is sealing the edge but is not pushing the tightend back into the pocket as well as the other two are. The RED circle shows the guard getting to the second level. The YELLOW circle shows a potential running lane opening that will need to be filled by Burns who is staring into it so he should recognize that he needs to fill the lane..
SEQUENCE BREAKDOWN 2
Artie Burns goes in to seal the running lane and in so doing take down Nebraska runningback Ameer Abdullah. Pierre sheds his blockers but does not have the angle to catch the speedy runningback. McCord realizes he does not have complete contain of the edge and starts to pursue Abdullah to the sideline. The inside running lane due to Artie Burns' pursuit has been cut off.
SEQUENCE BREAKDOWN 3
Burns misses the tackle. In so doing he rerouted Abdullah and he now has a new running lane: the sideline. McCord sheds his blocker and goes in pursuit.
Abdullah gains 7 yards to make it third and two. McCord on the tackle.
The point of the above breakdown was that the gap assignments, relatively speaking, worked. If Burns makes the tackle it's a two yard loss. However, he didn't and because Abdullah got rerouted to the outside he was able to break contain and get an above average run.
Personnel For Each Scheme:
For gap responsibility, it's just a matter of the overall scheme the coach wants to run for the defensive unit as a whole. If the team runs a 34 front then the defensive linemen are, more times than not going to be larger, hulking players. The defensive ends will be in the range of 6' to 6'4+ and 290 to 320 pounds. The defensive tackle or nose tackle will usually be around the same height requirements as the ends but usually heavier, around 310 to 330 pounds. For a 43 front the ends can be anywhere from 240 pounds to 275 and the defensive tackles can weigh in the low 280s+.
If you had to make an argument for specifics for gap responsibilities it'd probably be best to have larger linemen for the 2 gap type scheme and smaller, quicker personnel for the 1 gap. This is due to the fact that the larger players will be able to (in theory) take/dole out the punishment of the 2 gap responsibility and the smaller, quicker players can get after their rush lane (again, in theory) faster. However, it should be noted that coaches often have different plays that use both schemes so either type of personnel works for the type of gap responsibility, it really just comes down to what the coach prefers in the personnel types.
How this Affects Miami:
We won't beat the dead horse that is Miami running a 34 scheme in this piece but we will discuss how Miami has run the 2 gap lane responsibility and how it has fared (or hasn't?) in the past few seasons. Miami last season and heading into this upcoming season has the personnel for either scheme. However, last season and historically before that Miami specifically ran the 2 gap under Coach Al Golden and defensive coordinator Mark D'Onofrio. The results have been mixed to below average.
In the running game Miami has gotten better in holding opponents in check over the last two season. Miami averaged only allowing 137 yards on the ground which isn't too bad. Keep in mind, two of those games, TWO of those games (Nebraska and GT) Miami gave up 661 of the total 1782 on the season.
Through the air though, Miami at times was a sieve and one of the many reasons was the fact that the edge rushers never were able to get to the quarterback. Through the air Miami allowed 192 yards per game and totaled only 10 interceptions over 13 games. Their third down conversion rate was one of the worst in the country as well. The third down conversion rate is damning in the sense that more times than not these were passing downs and Miami simply couldn't get to the QB. The players were often quoted as saying "one person wasn't doing their job" or someone was "freelancing." That is one of the issues with 2 gap responsibility specifically. If you give a player too many responsibilities sometimes he can't fulfill them all on any given play, thus causing a breakdown. Now, is that always on the defensive line or pass rushers? No, but it can be a viable reason.
What Next Year (Possibly) Brings:
With the quiet (quiet?) addition of Randy Melvin as the new defensive line coach to the staff, Miami has added a veteran coach who brings both heavy experience in the pro and college ranks. He also brings with him a wealth of experience with 1 gap techniques for the defensive linemen he will be instructing. This maybe a relief for Canes fans to hear. Couple it with the fact that during the offseason besides visiting the Cowboys during their playoff run Coach Golden took a visit to the great northwest to pick the brain of Pete Carroll and his defensive staff on their defensive scheme (primarily 1 gap) and it looks like Coach Golden and the Canes maybe transitioning to a more aggressive, free flowing defense in 2016.
Of course, the results are yet to be seen but hopefully the Canes will employ a more aggressive and free thinking defensive line rush in the season(s) to come.