Hey, Kip here. You might remember me from Seahawks Draft Blog. I'm really tight on time this year, so I can't contribute terribly often either here or at SDB, but this is a topic I've had on my chest for a few weeks and since Brandon (Adams) asked me to contribute, I figured this would be a good place to start this topic.
When Pete Carroll brought his "Leo" defense to Seattle in 2010, it didn't officially change Seattle from a 4-3 to a 3-4 team, but in some ways, it was a step in that direction. Seattle's previous 4-3 defenses featured two pass rushing defensive ends who could perform reasonably well against the run, a pass rushing defensive tackle (known as the 3 tech), and a single gap run stuffing defensive tackle (known as the 1 tech).
Though technically still a 4-3, the "Leo" concept is a bit more unbalanced. The strong side (usually left) defensive end needs to be death to the run while also collapsing the pocket, but isn't expected to be a consistent pass rusher who generates sacks. The strong side defensive tackle, who manned the 3 tech in the previous defense, becomes a two gap player, something that is generally only found in 3-4 defenses. A two gap player does not shoot a gap, but instead directly engages a lineman and protects the gaps on both sides of him. The 3 tech role still exists, but was moved from the strong side to the weakside. Finally, you have the Leo position off the weak side end, and this is where almost all of the pass rush is generated from.
As with any scheme change, you will have players who benefit and others who suffer.
