Keep in mind, that this little overview is going to be relatively basic, as I am no football coach. However, I think it will be helpful to explain the basic concepts of the 3-4 defense so that my less football-savvy readers can understand the concepts of positional flexibility and zone blitzing.
Base defense
The basic defense, at first glance, appears simple:
Picture taken from Football.com
There are three down linemen - a nosetackle (also called noseguard), and two defensive tackles/ends. Also, there are four linebackers - two inside linebackers, and two outside linebackers.
However, the base defense is only the beginning. For every position but the nosetackle, there is a certain amount of positional flexibility required. That is, the players must be able to, on any given snap, play one of two (or even three) positions.
The nosetackle's job is to occupy two blockers and penetrate into the backfield. Noseguards are big and strong, usually bigger and stronger than the center they are matched up on. This size and strength advantage means that one of the offensive guards will have to help.
The defensive tackle/end is a hybrid position. In the base defense, the DT's job is that of a standard defensive end - off tackle run support, pass rushing, and outside contain. However, in many plays, one or both of the outside linebackers will drop down to act as a defensive end, and the DT is supposed to either become a traditional defensive tackle with run stopping responsibility, or in zone blitzes, drop into coverage (see zone blitzes below).
The outside linebackers, then, are another hybrid position. In the base defense, they have read-and-react responsibility for their zone. In a blitz, they can drop down into the defensive end position and become responsible for rushing the passer, outside contain, or both.
The inside linebackers are another hybrid. In the base defense, they have inside run support responsibility and zone protection against the underneath short passes like slants and crossing routes. However, the inside linebackers must also be big and strong enough to blitz the middle, taking on and shedding offensive linemen.
Zone blitzes
All of this positional flexibility in the front seven allows this defense to confuse the offense through the use of zone blitzes. Zone blitzes permit the defensive coordinator to bring pressure from anywhere, everywhere, or nowhere on any given play. For example, in a passing situation, all four linebackers could drop into zone coverage, and the three down linemen could rush the passer. Easy enough to react to and block by the QB and O-line respectively. However, the same could be achieved by rushing both outside linebackers and dropping the DTs into short coverage. Now, the O-linemen who thought they were blocking the DTs are out of position to pick up the outside linebackers. Result? sack.
On the other end of the spectrum, a full blitz could be brought, just like under the 4-3, 5-2, etc.
Where the zone blitz package becomes really effective is in between the two extremes. For example, the left inside and outside linebackers could blitz, and the DT drop back into zone coverage, leaving the O-linemen confused as to who to block. Or for an added wrinkle, the two linebackers AND the DT could blitz, with the other inside linebacker rolling over into short coverage on that side, and the opposite DT dropping into zone. Now, there are four rushers against 3 blockers on that side, forcing the O-line or running backs to shift on the fly and pick up the extra blitzer.
Summary
The 3-4 defense permits a team with athletic players to take advantage of their athleticism, rather than relying on strength. In a 4-3, the D-line is supposed to take on blockers and go after the ball carrier in the backfield, and the linebackers are responsible for run support and short zone/man coverage. The 4-3 matches up athlete for athlete, whereas the 3-4 creates mismatches.
The drawback of the 3-4 is that you could theoretically have a defensive tackle trying to cover a receiver on a crossing route - mismatches can go both ways. It relies on creating disruption and throwing off timing to be successful.
Which defense is better for any given team depends on the personnel. If you have a team full of prototypical players at their position, the 4-3 is better. However, if you have a bunch of players that would be 'tweeners - equally capable of playing two positions, but not prototypical in either - the 3-4 is better.
Notre Dame's personnel this year is particularly suited to the 3-4. Tomorrow's post will explain how I expect this all to shake out.
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