I've always wanted to write an article on the Chargers' defensive secondary scheme. Unfortunately, we haven't had access to enough film to make it a worthwhile effort (though that should change this year), and it's a bit pointless with the yearly turnover at DC anyway. Jeff's excellent article explaining the basics of Air Coryell inspired me to take a different route, though, and try to provide an explanation of how the 34 defense works. I'm going to try to avoid deviating into explanations of the 43 and how the two differ because I don't want to write a book. I apologize in advance for the crappy diagrams; I only have Photoshop to work with here and I'm not any good with it
First Things First
The defensive line is the base on which the rest of the defense is built. In the classic 34, the defensive line play is focused on 2-gap technique. That means each lineman is responsible for two gaps, as defined here:
In the classic 34, the Nose tackle plays "0 Technique" directly over the center, and is responsible for both A gaps. The DEs play "4 technique" directly over the tackles, and are responsible for the B and C gaps. This is why many players have trouble moving from the 43's 1-gap scheme to the 34; they have to be able to handle the lineman in front of them, and control the gaps to either side. The system leaves the guards uncovered, unless the DL are good enough to require double teams. The job of the DL in the 34 is to occupy blockers so the linebackers can make tackles and get sacks, as we'll discuss later.
The modern 34 is somewhat different, and is more of a hybrid of the 34 and 43 philosophies. This is in some part due to the difficulty of finding enough huge, athletic guys that can play 2-gap technique (Parcells' "Planet Theory"), and in part to generate more pressure.
In the modern 34 schemes, DL are used more like they would be in a 43, often playing 1-gap technique and shading to one side or the other, often lining up over a gap as opposed to a lineman. The entire DL is commonly shifted to the strong or weak side ("over" and "under"). Stunts and other trickery are used in a further effort to generate pressure.
An "Under" shift:
The Second Level
All of this serves to free the Linebackers to make plays. The linebackers are the whole point of the 34; using big defensive linemen to occupy more blockers allows you to put four fast, athletic guys on the field to keep up with the increasingly athletic offensive "skill" players and to take advantage of the increased focus on speed in college football. There are two classes of 34 linebacker: Outside and Inside.
Outside linebackers are fairly well understood by most fans. Their primary job is to get pressure on the Quarterback. They generally line up off of the OT's outside shoulder. They occasionally drop into coverage and are often pretty bad at it, but their primary job in coverage is often more to confuse the QB about who's blitzing, as we'll see in a moment.
Inside linebackers traditionally were assigned to take on the guards left uncovered by the classic 34 scheme. Thus they were big, bruising types. As the game has changed, coverage has become more important and speed has developed into one of the primary characteristics of an Inside Linebacker. They still have to be able to attack the gaps on run plays, but they need to be able to cover TEs as well. The Inside Linebackers are often the big tacklers in a 34, as they are right in the middle of the defense and ideally are always around the ball.
One of the defining plays of the modern 34 is the Zone Blitz. The concept is simple and not unique to the 34: some linebackers (or safeties, or corners) blitz while others drop into zone coverage. It is particularly well-suited to the 34 because the extra linebacker can both drop into coverage or blitz, and the QB doesn't know which. In a 43, the blitzing LB leaves a hole in the coverage, and the QB knows that a DE or DT dropping back is easy pickings. In the 34, there are four potential blitzers who are still athletic enough to play coverage, if not particularly well at least better than a lineman. They can all show blitz pre-snap, and any one can drop back or delay his blitz. This is why a guy like Shaun Phillips, who is a good but not great pass rusher, is still highly valuable as a 34 OLB due to his coverage skills.
A simple 34 zone blitz (though it's more common with 5 rushers):
Some defenses go crazy with the Zone Blitz, mixing in man coverage on one half of the field and zone on the other, dropping all LBs back and blitzing with both safeties, and every other combination of coverages you can think of, but the creative stuff serves primarily to confuse the Quarterback and keep him from getting comfortable, making the basic variations more effective. Anyone can be a rusher, and anyone can be in coverage.
The Last Line
Another important aspect of the Zone Blitz is that it still provides for good coverage, as opposed to all-out blitzes that leave CBs alone in man coverage and risk a big play downfield if the rushers don't get home in time. Zone coverage calls for the defenders to watch the quarterback and play the ball, as opposed to man coverage which requires the defender to play the receiver, with his back to the QB. Zone defense is an easy way to generate turnovers because the defenders are watching the ball and have a better shot at making a play on it. The zone blitz increases that effectiveness because the QB is confused about who's really in coverage, and is hopefully getting pressured by the rush.
Here's a full-field zone blitz diagram. The Weak-side ILB fakes a blitz and drops into the flat. Again, any of the LBs or safeties could be showing blitz before the snap.
Plays like this are what made Troy Polamalu and Ed Reed famous. As the deep safety, they have time to read the play and react to the ball, getting there in time for an interception or to break up the pass. It's effective because their respective defensive fronts are able to get to the QB and force a quick decision, which is often a bad one when Ray Lewis is coming for your head.
The secondary play is very similar between the various defensive fronts, so I'll provide a quick primer on the various roles.
The Chargers have typically played more man-to-man coverage, at least in part due to the skill sets of the players in the secondary. Last year, Eric Weddle was put in that deep zone safety role and he took advantage. Unfortunately, we also saw Jammer and Cason playing zone and getting beat at it. Man-to-man coverage is simple in concept, but has several variations that call for significantly different skill sets.
Off-man coverage calls for the corner to line up 7 to 8 yards deep. He watches the receiver's hips to determine the break, and turns his own hips within 5 yards. He has to get underneath the receiver to break up the pass. This style is vulnerable to very fast receivers and short slants underneath the cushion. It's also possible to defeat off-man with a convincing double-move or juke that forces the corner to commit his hips on the fake.
Press Man coverage calls for the corner to get his hands on the receiver and disrupt his timing. He stays lateral and keeps himself between the receiver and the goal line. It's often difficult to get both hands on the receiver, so the corner has to determine the release and use the off hand to jam the receiver while turning his hips upfield. We all saw how this can be defeated by a big, strong receiver in last year's game against the Jets. On the third TD, Cason used the play-side hand and Burress was able to shake him off and run past while Cason had to turn his hips around and try to catch up. After the jam, the corner has to initially maintain underneath position to protect against short passes, then prevent separation on a deeper route. Which he plays, and when he turns upfield, are determined by the situation and the receiver.
The role of the safeties in the Chargers defense has changed several times over the years. Typically, the Free Safety's job is to read the play and either backpedal to provide deep pass coverage or come up to tackle the runner. If playing deep coverage, he'll attempt to read the routes and stay behind the deepest receiver. This is rule number one for a deep safety: never let a receiver get behind you. It's tempting to undercut the route and go for the interception, but if you fail it often goes for 6 points. The deep safety will provide over-the-top coverage for a corner on a deep route, but has to commit quickly in deciding which side of the field to play. He does this by knowing his corners and reading their play at the line.
The Strong Safety typically plays on the strong side (no kidding) about ten yards deep. He is often assigned to cover the TE, but also blitzes and covers slot receivers, and is a primary part of run defense. The Strong Safety's reads are often simpler because, if he isn't assigned to deep coverage, there is less risk in a mistake. Thus he can watch the linemen stand up or stay low and react quickly. The FS has to be more careful to look through the line to the backs.
Of course, this all changes with Nickel and Dime sets. I'm not even going to get into that right now because we really don't know what Coach Pagano's going to do just yet. We some some interesting looks in the preseason and I may get around to explaining them at some later time. Feel free to add anything I may have missed in the comments; I only really know anything about the secondary stuff.