Several weeks ago, we reviewed the basics of gaps and techniques on defense.

You'll recall I compared football to a chessboard--techniques are where the pieces line up, while gaps are the responsibilities of the pieces. The big difference is that in football, the pieces (defenders) can line up in a different spot on every play.

Now let's put that knowledge to work in the Broncos' current 4-3 system under Dennis Allen.

Allen runs the two prominent forms of the 4-3, the Under and the Over, although thus far in the season, he's favored the Under scheme.

You're about to find out why.

A Quick Review

Remember, Allen likes to run a one-gap 4-3 system. There's a good reason for this. In technical terms, it's much easier for defenders to use their quickness off the ball to defend one gap than it is for defenders to read and react while holding responsibility for two gaps. For a non-technical explanation, we turn to Brian Dawkins:

Attack. Attack. Attack. Attack. Not sit back. Not wait. Not two-gap. Beat your cat, let's attack. In crunch time situations, we're not going to sit back, we're going to attack.

Sounds good to me. In fact, the 4-3 one-gap system is perfect for a personnel-challenged team like Denver. It takes neither a defensive guru nor consecutive years of winning the NFL draft lottery to make your defense respectable. Unless you've got some serious bad asses on your defensive line who can stack and shed like Hulk and Wolverine, the 4-3 one gap system is the fastest way to turn around any defense, especially in the running game.

But there's even more that the 4-3 can do if it's really serious about stopping the run.

4-3 Over vs. 4-3 Under

Before we heap praise on Allen's use of the 4-3 Under, let's take a look at what differentiates it from an Over scheme. First, here are the gaps and techs of a 4-3 Over:

This isn't going to be earth shattering by any stretch. As we've described, in a one-gap 4-3 system, each defender is responsible for...you guessed it, one gap. The defensive linemen from left to right (from the perspective of the defense) each play a 9-, 3-, 1- , and 5-technique and have one-gap responsibilities. The Will, Mike, and Sam also have one-gap responsibility, and are lined up as even techniques. This is primarily because this puts them in a better position to flow to the ball both strong and away as they read and react to a play.

So why is this called an Over? The answer lies in the shift of the two tackles and the positioning of the Sam linebacker, which we'll get to in a moment.

First, though, you may be wondering what the red circles are for. These are called bubbles in coach speak, but don't get bogged down by jargon. Visually, these red circles/bubbles represent space and vunerabilities in the defense. They are also the first place you'd want to attack in the running game as an offensive coordinator, for two reasons: First, given a preference, one wants to attack an area of weakness rather than one of strength. Attacking a space that is currently unoccupied and relies on a linebacker filling a hole is, in general, a better idea than attacking a hole or gap in which a defender is known to be already. Second, one likes one's chances against lighter defenders like linebackers.

Notice that in the Over scheme there are three bubbles to attack.

Another point I should make is that in some schemes you'll see the Sam and the Will flip. In the Tampa scheme of the 90s, for instance, you saw the Sam lined up on the open side of the formation. But for now, and for the purposes of simplicity, we'll keep our Sam linebacker (as the Broncos have been in the Over) in his natural state of bliss on the strong side of the formation.

Now let's take a look at an Under scheme for comparison:

Notice the change between the two? The under tackle has shifted from the strong-side 3-technique in the Over formation to the weak-side 3 technique in the Under formation. Similarly, the nose tackle has shifted from the weak-side 1 technique in the Over to the strong-side 1-technique in the Under.

The other major change is that our Sam linebacker is now playing at the line of scrimmage on the strong side of the formation as a 9 technique. This creates the appearance of a 5-2.

Not only does this change add another defender to the line of scrimmage, it also--magically one might say--creates one less bubble for the defense. Where there once were three bubbles in the Over formation, there are now just two bubbles in the Under formation.

As an armchair defensive coordinator, you are now qualified to answer a question:

1) Which version of the 4-3 do you believe would be stronger against the run? Against two-back formations?

Clearly, the Under is a stronger run defense. It gives the offense less bubbles and puts more pressure on the line of scrimmage. It also has the added benefit of having the potential to generate a pass rush from the Sam linebacker if, for some crazy reason, you possessed a Sam linebacker that happened to be one of the quickest linebackers in the NFL. Down-and-distances like 2nd-and-5 now become much more fun as a defensive coordinator. You can play your "Under" scheme in anticipation of a run, but just in case the quarterback does pass, you're not caught without the potential for pressure.

Obviously, the Broncos have been using this formation a lot this year, and they had it in mind when they drafted Von Miller. It's also a reason why they didn't feel the need to go out and overspend in free agency for a guy like Brandon Mebane. Playing the one-gap 4-3 Under with a freak like Miller gives you both the ability to cover slight deficits in run personnel (Miller's run defense has been nothing short of amazing thus far) and the ability to generate a pass rush with sometimes-minimal effort.

In a division like the AFC West, this is even more critical. The Raiders, Chargers and Chiefs all use multiple-back sets; giving them less bubbles in which to find space is Mission One, even before addressing personnel deficiencies. Adding Miller to the mix just ups the ante.

Of course, there are weaknesses in the 4-3 Under. One of those is how to handle extra receivers out of the backfield. Do you roll into Cover-1? Or do you risk a linebacker? To these questions we'll return another time. Suffice to say that you'll continue to see the 4-3 Under used by Dennis Allen and the Broncos with their current personnel. It's a primary strategy in turning the corner with this defense.

How to Spot It

If you choose to simply follow the ball, that's perfectly fine. Things develop quickly, and following the ball is sometimes a more pleasing emotional experience. If, however, you want to try and go a little deeper, as usual, we always try to give you tips for spotting formations, personnel packages, and other oddities as quickly as possible after the huddle breaks. As a viewer, you've often only got two or three seconds before the ball is snapped to figure it out.

The fastest way to tell whether or not the Broncos are in a 4-3 Under or Over position is to look at the under tackle and the Sam linebacker. If the under tackle is shifted to the weak side in a 3-technique and the Sam is on the strong side at the line of scrimmage, you're looking at an Under formation. If not, and you see three linebackers with five yards of depth, you're looking at an Over formation.

The quickest and dirtiest way to tell (although I'm not promising it won't be a nickel 4-2-5) is to simply watch Von Miller. As the Sam, he's a dead giveaway in the 4-3. It just so happens, in Miller's case, he's also the most deadly.

TJ Johnson can be reached through telegraph, ESP, Spanish interpretor, or via email: tjthedudejohnson@gmail.com. Follow him on Facebook and Twitter if you want to see him mock "the man." He assumes you are following It’s All Over Fat Man on Facebook and Twitter, but if not, you're a nihilist, man.