Since Monday, much of the talk here and elsewhere has been the performance (or lack thereof) of the defense against the Broncos.  One argument for showing some patience is that the team is still adjusting to the 3-4 and will improve through the preseason.  Whether you buy that or not, is not important for this particular post.  Rather, I wanted to address the details of the 3-4 defense.  Some of you probably know a lot about it, some of you probably know little to nothing about it and some like myself know enough to be conversant, but not much more.  So I decided that rather than spout with what I know, I'd do some research and tell you what others know about the 3-4.  This switch in defenses is a huge change for the 49ers and will impact the way they play defense this year and for years to come.  It'd be nice to know what it's all about right?  For those who have more detailed knowledge, feel free to throw in your 2 cents on the subject.

The 3-4 requires a huge nose tackle and two large defensive ends along the defensive line.  In the two-gap version, the three down linemen occupy two gaps each (duh) as seen in the diagram.  This is meant to open up holes for linebackers to make more of the plays.  Your defensive linemen will have less sacks, but are essential to the success of the linebackers.

At least one of your linebackers will usually be blitzing from some angle and one of the pluses of the 3-4 is that this flexibility in who blitzes can easily create confusion for the line and help create pressure on the quarterback.  The 3-4 will show us what kind of stud Manny Lawson can really be as he will definitely get a lot of chances to rush the quarterback and load up on sacks.

One of the weaknesses of the 3-4 can be the attempt at filling the various gaps by just three defensive lineman.  A team will not always fill up the various gaps, which can decrease QB pressure.    The zone blitz is one way to counteract this by shifting your blitzers as described above.  I still remember the Carolina Panthers running this against us quite often there first couple of years.  Dick LeBeau is widely considered the inventor of the zone blitz and I'd expect to see some of it this season by the 49ers.  If the team can't get pressure from that linebacker, it could just end up being a weak repeat of last year's ineffective pass rush.

Our inside linebackers will need to be solid against the run and be able to throw off offensive linemen on occasion.  Ray Lewis had an interesting quote (via Baltimore Sun) when the Ravens switched out of the 3-4:

I definitely will not pretend to know a whole ton about the 3-4.  I do know that having Aubrayo Franklin on the sideline is not good for developing chemistry on the defense.  The nose tackle plays a huge role, both figuratively and literally.  If you look at past successful 3-4 defenses, you saw nose tackles like Ted Washington and Casey Hampton just blowing up the interior of the offensive line and stuffing it up completely.  It may not be until he gets completely healthy and out on the field that we know just where this defense is.

I'd like to open it up to discussion.  Below are links to the sources where I found a decent amount of information, and feel free to throw out any other sources, kind of like a link dump for this specific issue.  The more we know about the 3-4, the better discussions can be in the future.

Wikipedia - Everybody loves wikipedia

Football 101 - An in-depth football site with quite a bit written by a former scout

Coachz.net - Good graphic with basic descriptions

Random Raiders fan site - A quality description