The 3-4 defensive mission statement:

The 3-4 defense is all about confusion.  Using strong linemen to occupy the offensive line at the point of attack, athletic linebackers are able to attack from various angles to pressure the quarterback or stuff the run.
The 4-3 usually relies on the down linemen to penetrate.  This penetration by the linemen disrupt running plays & put pressure on the quarterback as well.  The defenders will line up in gaps and attack those holes when the play starts.  In the 4-3, the men in front of you are simply obstacles to get around if you’re a linemen.  You want to ‘defend the run on the way to the quarterback’ in most 4-3 defensive schemes.  The scheme, as far as the down linemen are concerned, is about speed, agility, and athleticism.

However, the 3-4 is based on strength, grit and leverage if you’re a defensive linemen.  Instead of lining up in a gap between two linemen (often referred to as a ‘one gap’ defender), all 3 linemen play what is called a ‘two gap’ style of defense.  Instead of lining up in or near a gap, being responsible for anything that comes through that hole, a two gapper lines up directly in front of the lineman he is to attack,and is responsible for controlling both the gap to the left and the right of the man on offense.  This is obviously much more physically demanding, and requires players who are much larger and stronger than their 4-3 counterparts.  A lot of the time, 4-3 defensive tackles end up playing 3-4 defensive end.   As you can see, the responsibilities for each lineman essentially double in the 3-4:

Occupying multiple offensive linemen like this open up all sorts of opportunities for the linebackers and the defense. To make the 3-4 engine ‘go’, the nose tackle (the middle defensive lineman) must command a double team.  This will open up a gap for blitzers to come through.  If the nose tackle doesn’t command a double team, a lot of what follows is moot.  Below is an example of an offensive blocking scheme against a 3-4 line:

As you can see, the offensive guard and center both attack the nose tackle, and the offensive tackles both engage the defensive ends.  There is on ‘free’ blocker for the offense, who kind of chips off of the end.

Traditionally, teams will rush the 3 linemen and bring one or maybe two linebackers on a blitz, while playing zone coverage behind the blitz.  In other words, the players that do not rush the passer will stay back and guard and area of the field, anticipating a quick throw from the quarterback under duress from the unblocked rusher.  Let’s presume the offense is running a pass play, uses the above blocking scheme, and the defense called a simple ‘overload blitz’ coming off of the edge.  That would look something like this:

As you can see, the blitzers overwhelm one side of the field, with the players behind the play dropping back and waiting for the quarterback to throw (if he has time).  Because the nose guard has occupied two blockers, holes develop that allow linebackers to slip through, causing all sorts of chaos before the offense can really even get the play started.  However, a lot of the time offenses can do an OK job of defending against just one or two free blitzers.  That’s where something even more disruptive comes into play.  Zone blitzes can be run out of a 4-3 or a 3-4, but it’s especially effective when you have one extra linebacker to work with.  Zone blitzes usually take at least one person that you expect to rush, drop them back into pass coverage, while overloading one side of the field with rushers.

Let’s assume again that we’re working from this basic blocking scheme:

Instead of all 3 linemen rushing, the end on our left will drop back into coverage, and the 3 linebackers from that side of the field will blitz.  2 of them will rush immediately, while one will delay, waiting for the offensive linemen to react to the first wave, and then running around those blockers.

These sorts of plays are textbook 3-4, and what make the entire scheme so disruptive.  The problem of course is getting the right players.  You need big, strong defensive linemen.  You can’t take a play off, you have to come to fight every play, and you have to work to occupy multiple linemen on every play.  You also need linebackers who are fast, strong, and smart.  The middle two linebackers especially must be larger than normal linebackers, as they are taking on offensive linemen more often than their counterparts in a 4-3 (4-3 linebackers usually are in pass coverage, or guarding tight ends or running backs, but rarely engaging linemen directly).

If, and that’s a big if in the college game, you’re able to get the right players in place, you can really cause problems for opposing teams.  It’s very similar to Paul Johnson’s offense in that teams just can’t effectively prepare for what they’re going to see in such short time, and it usually will lead to breakdowns.  These breakdowns should become turnovers, and help Tech win a lot of games with an already potent offense.

I’ll go into more detail about GT’s exact implementation, the types of players needed and if Tech actually has them before the first game next Saturday.

By all means, let me know if anything didnt’ make any sense or you’d like me to go into further detail.