Over at Grantland Chris Brown of Smart Football has a great article on Vince Wilfork and the Patriots D.
The main focus on the article is how Belicheck makes his Hybrid 3-4/4-3 work. The biggest part of this article for Michigan fans is the part where Chris Brown explains the history of the formations and how we arrive at Michigan's 4-3 Under front. Especially since so many people think we're recruiting for a 3-4 (Guess what, we're NOT! http://mgoblog.com/content/mailbag-3-4-switch-again-kenpom-basketball-leaders-more-you-know, http://mgoblog.com/category/tags/last-time-we-are-not-switching-3-4)
this is good stuff. He explains the main differences between the techniques required in the 3-4 vs the 4-3. I tackle the (lack of) personnel differences between a 4-3 Under and a 3-4 here:
But here are the juciest bits from Chris Brown (the whole article is definitely worth a read):
These 4-3 and 3-4 teams typically differ in a key respect: which "technique" their defensive linemen use. Usually, teams must commit to one technique or the other, as each choice has all sorts of other implications for the defense.
And the first question for a defensive lineman is always, Am I playing a 2-gap technique or a 1-gap technique?
"Gap" refers to the area between offensive linemen. A 1-gap technique is just what it sounds like: The defensive lineman lines up in front of the gap he is responsible for and his job is to attack and control it. If nothing else, a defender must not allow a runner to go through his gap. While defensive linemen attack their gaps, the linebackers behind them are responsible for their own gaps. These are the defense's "run fits," meaning how they fit into an offense's blocking scheme to take away running space.
Pretty much we're going to run the 1-Gap 4-3 Under because it's a lot simpler to teach. 2-gap systems like the 3-4 are a lot harder to run. Just go read the article, it's great.