For several years, the SEC had a reputation as a conference whose defenses could squash any offense in the nation. The spread offense air raiders might come and pillage other conferences, but in the Southeastern lands, old-school offensive tactics were walled in.

Then Urban Meyer and Tim Tebow unleashed a 29-6 league record over four years, and the narrative began to evolve. The spread-option took hold in schools in Mississippi and Alabama.

In a move similar to that of a collapsing empire, the SEC expanded and welcomed the barbarians inside the walls, adding the wide-open Big 12's Texas A&M and Missouri. With a new air raid coach and the instant legend of Johnny Football, the Aggies pillaged defenses and shattered the mythos of SEC invulnerability. In a 2013 season in which high-scoring slugfests became the norm, three different SEC programs with "spread" offenses ranked in the top 20 nationally: A&M (4th), Auburn (6th), and Missouri (19th).

However, while programs like Alabama have seen their invincible defensive reputations take hits (the Tide surrendered 1,046 yards in just two games against Texas A&M), one program has maintained a defense that's sheltered the league from being totally taken over by the spread: the LSU Tigers.

Included in the Bayou Bengals' resume of smashing spread teams are handling Chip Kelly's Oregon, air raid West Virginia, and Bobby Petrino's "pro-spread" Arkansas. They forced the two worst games of Manziel's college career. They've yielded an overall conspicuous lack of games in which spread offenses lit up the scoreboard.

Of course, few will ever agree on a definitive list of teams that run spread offenses.

But if we define the power-conference spread offenses LSU has faced over the last six years as Gus Malzahn's Auburn, Mississippi State, Hugh Freeze's Ole Miss, Oregon, Dana Holgorsen's West Virginia, Petrino's Arkansas, Texas A&M, TCU, and Clemson, then we can figure the Tigers have given up an average of about 20 points against some of the country's highest-octane offenses. While the Rebels had the Tigers' number last year and some have put up meaningless yardage in LSU wins, the track record is secure.

So what are the secrets to defensive coordinator John Chavis' approach? How is it that LSU consistently outperforms other big-time programs with defensive reputations in stopping these offenses? The answer has multiple layers.

I. The foundation: Personnel choices

You can't talk about what makes LSU's defense so effective and versatile without first acknowledging that Les Miles assembles some amazing athletes in Baton Rouge. Part of this is simply a luxury of being the only power-conference program in the state that has the highest rate of DI athletes per capita in the entire country.

However, LSU selects certain types of players for its defense. They prize length and athleticism along the defensive line, assembling players with ideal NFL measurables that don't stick around for their senior years, yet aren't terribly missed.

At linebacker, the Tigers tend to go for athletes in the 220-to-240-pound range who can hold up well enough in coverage, change direction on the run, and shoot into the backfield with explosiveness on a blitz or after diagnosing a play.

And in the secondary, despite relying heavily on man coverage, the Tigers rarely make exceptions in their quest to find the rangiest athletes available. Adding up all their defensive backs taken in 2015 (thus far) back to 2010, you'll find 17 of their 21 DBs were at least six feet tall. Exceptions were made for Tyrann Mathieu and Micah Eugene for obvious reasons, as both players had extraordinary quickness.

When it comes time to play a spread team, LSU has no shortage of tall, physical pass-rushers and defensive backs to shrink the field back down when spread teams try to use wide alignments to spread them out. As Sport Science noted of the legendary 2011 LSU secondary ...

... the field shrinks down when athletes of this caliber are zeroing in on an offense.

Additionally, LSU doesn't hesitate to play their underclassmen early and often either in sub-packages or in relief of the starters, which helps maintain a never-ending wave of relentless effort.

II. Manning the wall: Choosing leaders

The secret to playing effective defense in the modern era involves having experienced players at key positions who can line their teammates up against tempo offenses and understand how to play against different offensive sets.

While offenses test the physical abilities of defensive linemen and corners, they attack the minds of linebackers and safeties. They use concepts designed to confuse them or put them in conflict.

LSU's defense is actually built around the leadership of safeties and linebackers.

While LSU is known for having NFL corners and defensive linemen, its defense is actually built around the leadership of safeties and linebackers. In 2012, will linebacker Lamin Barrow, mike linebacker Kevin Minter, and free safety Eric Reid held down the middle of the Tiger defense and stayed on the field in every package.

For the 2013 unit, the team was anchored by linebackers Lamin Barrow and D.J. Welter along with strong safety Craig Loston. The 2014 defense will be led by Welter at mike linebacker and Kwon Alexander at the will, along with a need for free safety Jalen Mills or another Tiger veteran to step up.

If the players in the middle can't recognize what an offense is doing and help the rest of the defense adjust and get lined up, the defense's ceiling is limited. While the Tigers might rotate in exceptional athletes along the lines and at corner every season, their ability to develop veterans to man the safety and linebacker positions is key to their defensive structure.

In 2011, Oregon averaged 537 yards against 13 teams not named LSU -- but managed 335 yards and four turnovers against the Tigers. Ronald Martinez, Getty

III. Formations: Building sub-packages

In 2011, it quickly became apparent that LSU's nickel package was the most talented personnel package in the entire nation. With the Honey Badger roaming the middle while Tharold Simon and Mo Claiborne locked down the edges, the defense was a swarming pack of wolves that could pressure and account for anything an offense wanted to throw at it.

Since Mathieu left, the nickel package has still been a big part, although Chavis' has also utilized quick linebackers like Alexander at the sam linebacker position and avoided using sub-packages unless an opponent's passing game required playing a DB.

With LSU returning several backs who played in 2013 and with sam backer Alexander moving to will, LSU might utilize the nickel more against spread sets. Ultimately, the 4-3 base defense is flexible, to accommodate whichever personnel packages prove to put the best players on the field in a given year.

The major sub-package that illustrates differences between LSU and most other teams is its mustang package:

The mustang comes into play either against four- or five-wide offensive packages or in obvious passing situations. The safeties (F=free safety, $=strong safety) and linebackers (W=will linebacker, M=Mike linebacker) remain on the field for leadership and versatility, but other personnel changes. The nickelback comes on, and if he's a starting corner or safety in the base 4-3 package, then his backup at the previous position comes in.

LSU generally plays one of its defensive tackles at the nose position while leaving its defensive ends aligned across from the tackles. The dimeback player comes in to play opposite the nickel.

In terms of personnel, it's a 3-2-6 package (3 DL, 2 LB, 6 DB). Before the snap, those players are lined up as they would in a 3-4. After the snap, this formation can morph into a large number of different calls.

Besides the disguises available to the defense in the mustang package, the major advantage is that it allows LSU to take advantage of a roster full of six-foot, 190-pound athletes who have incredible speed. Check out their dimeback Dwayne Thomas blitzing Johnny Manziel:

Thomas' speed allows him to pressure Manziel. He forces Johnny to his left, which was a priority for LSU in this game. After the elusive Aggie escapes, Thomas is still able to race him to the edge and force one of the most difficult throws you'll ever witness.

In this case, it's still not enough, and A&M has to settle for a field goal. It's unlikely that even a very good defensive end or linebacker is able to keep up with Johnny on the edge like that. But a DB can.

A big part of LSU's success against A&M over the last two seasons was based around flooding the field with athletes who could run with Manziel. By constantly forcing him left and and having enough speed on the field to corral him, the Tigers were able to survive his improvisations.

IV. Tactics: Anti-spread schemes

Chavis' 4-3 defense is pretty standard and similar to what you see at other programs across the country. The Tigers rely mostly on the over front, which gets the ends on the edge and safely away from double teams, and they use a standard combination of quarters coverage and single-deep safety coverages.

Where LSU is different from other teams and where they draw some anti-spread advantages are in the way they use robber coverage, press coverage, and their famous mustang.

Like many other teams, LSU typically responds to a trips set (three receivers on one side of the formation) with a particular type of cover 1/cover 3 called robber. The difference for the Tigers is that their strong safeties can play in soft coverage and either break on the ball or arrive at the line of scrimmage in time to make plays against the run game.

Before the snap, it's not necessarily obvious to the quarterback if the defense will play a variety of cover 4 with the strong safety dropping deep and the mike linebacker covering the third receiver, or if that strong safety will drop down. Even after the snap, the safety might not sneak too close to the line, leading to confusion for the QB about how aggressively he'll break on throws over the middle.

LSU might also drop its safeties down into this position on their blitzes, where they are trusted to make the shallow middle a no-fly zone while being reliable tacklers on the edge against the run.