The Falcons had an athletic, active defense under Brian Van Gorder that successfully kept teams out of the end zone most of the time, but was prone to massive breakdowns and long stretches of soft play. Under Nolan, the defense was still prone to those meltdowns—we saw it in the playoffs—but it was also an opportunistic unit that had serious success against some of the league's best quarterbacks.
The question is, what makes it work?
As I see it, Nolan's defense with the Falcons relies on four tenets. I'm going to briefly break them down, and then I'll turn it over to you for further discussion.
Some of the league's best quarterback anticipate what the defense will do, allowing them to make changes at the line of scrimmage and stay one step ahead of the D. This is less effective when the defense is continuously moving before the snap, making it difficult to predict blitzes or who will be dropping into coverage.
The most spectacular examples of how effective this can be came against Drew Brees and Peyton Manning a year ago, when Nolan's defense was constantly moving before the snap. Manning struggled mightily to account for every defensive back, and those defensive backs were able to keep an eye on Manning, anticipate his throws and pick him three separate times. It's not likely that Manning, one of the best to ever play the game, tosses all three of those if his timing and ability to anticipate isn't thrown off by eleven guys moving like a puzzle before he receives the snap.
The biggest problem is that smart quarterbacks begin to learn from this as the game wears on. If you can force several turnovers in the first half, however, you've put yourself in an excellent position to win football games. With Cliff Matthews, Kroy Biermann, Osi Umenyiora and others cross-training, we've only seen the beginning of those mighty morphin' fronts.
Okay, so you can confuse quarterbacks by changing things up at the line of scrimmage and moving pieces in space. All the confusion and anticipation in the world do you no good if your players can't get in front of the ball.
Athleticism is key. The Falcons don't have the fastest team defense in the NFL, but generally speaking they're quality athletes with good closing speed, which is especially important for the secondary. You can whir around the field, wreaking confusion upon receivers and quarterbacks, but once the ball is in the air or in the running back's hands you have to be where the play is.
Robert McClain is an excellent example of this kind of athlete. The 2012 breakout player flashed the kind of closing speed and on-field awareness that deserves to be highlighted on his pick of Peyton Manning. Wiliam Moore, who victimized poor (?) Drew Brees on more than one occasion, is another example of a guy who can make a few yards disappear in a hurry.
Without athleticism, this D doesn't tick, is what I"m saying.
So you've got all these athletes and you want them moving all over the field to confuse offenses, jump routes and offer up surprising coverages. For this to really work, you want your guys to be able to play multiple positions and pick up different responsibilities.
This can be as simple as having linebackers who can cover and play the run as needed, or as complex as bulking up Cliff Matthews so he can play defensive tackle and defensive end as needed. This versatility can be valuable when you've got Osi Umenyiora able to hunker down on the line or stand up, adding to the uncertainty. When you don't have the biggest, most physical defense in the NFL—and the Falcons don't—every split second of hesitation and moment of doubt matters.
Versatility also ensures that injuries or ineffectiveness at one position don't totally cripple your defense.
Stop The Pass
How many times have you heard that it's a passing league? This may mark the 700th time this week, for all I know. Clearly, though, stopping aggressive passing offenses is a huge priority.
Nolan's comfortable with that. The Falcons went with nickel packages well over 50% of the time, taking Akeem Dent off the field and replacing him with McClain. When you're facing Drew Brees, Colin Kaepernick, Russell Wilson and others all season long, this makes a ton of sense.
Is Nolan perfect? No. This defense may still have trouble with the run, and it's never been a great tackling team by any stretch of the imagination. Combine these four ingredients, though, and you have a defense that isn't going to be particularly fun to play against, especially for opposing quarterbacks.