Ed Reed and Troy Polamalu seem to have supernatural powers — they’re everywhere on the football field at once, omnipresent demigods determined to knock your chinstrap off. Their range of skills is remarkable: total tackles, interceptions (Reed led the NFL in picks in 2010 despite playing only 10 games), and even sacks (Polamalu owns the record for most sacks by a safety in an NFL game, with three). They play slightly different positions — Reed is a free safety and Polamalu is a strong safety — but that distinction means little to opposing coaches and quarterbacks. To them, Reed and Polamalu are men of mayhem, hungry for prey. Their success is a credit to their talent and work ethic, but also the result of defensive strategies that have helped them make their marks.

Football defenses have been reacting to offenses for more than a century, and there is very little in today’s game that wasn’t around 50 years ago. Indeed, almost all modern NFL defenses are indebted to the 4-3 defense — referring to four defensive linemen and three linebackers — that Hall of Fame coach Tom Landry invented while serving as defensive coordinator for the New York Giants in the 1950s. This pro-style 4-3 defense continues to evolve — along with its cousin, the 3-4 — but Landry’s basic scheme of a dynamic seven-man front supported by four flexible secondary players remains to this day.

For the sake of brevity, allow me to oversimplify some history and jump forward a few decades from the inception of Landry’s 4-3: By the mid-1980s, offenses had gained an upper hand on the formation. Defenses struggled to simultaneously deal with power football — that of fullbacks, tight ends, and pulling linemen — and increasingly efficient passing offenses like the one designed by the San Francisco 49ers’ Bill Walsh. The best-known (and, for a time, the most effective) response to these developments was the “46” defense implemented by Chicago Bears defensive coordinator Buddy Ryan. The theory behind the 46 was that offenses seized the advantage because defenses let them dictate terms. For 30 years, defenses more or less tried to match and mirror offenses based on personnel and alignment, but they couldn’t keep up. Ryan planned to negate this advantage by force — the 46’s simple guiding principle was to kick ass.

The 1985 Bears, using Ryan’s 46 in increasing doses throughout the season, fielded what may have been the greatest defense in NFL history en route to a Super Bowl victory. Yet the name “46,” unlike the 4-3 or 3-4, didn’t refer to the defensive alignment. Instead, it referred to the guy who wore jersey no. 46, Doug Plank, Chicago’s clever and feisty strong safety. In the 46, Plank was moved out of the pure secondary and became a kind of linebacker. This allowed the Bears defense to put more defenders on the line of scrimmage than the offense could block on both runs and passes. Instead of matching the offense, the 46 sought to overwhelm it. And that season, the Bears did just that.

But, as they always do, offenses adapted to the 46. Spread formations and quick passes became serious challenges for defenses. Even Buddy Ryan, who became head coach of the Philadelphia Eagles in 1986, found his beloved 46 defused by spread concepts and had to cease using it as an every-down defense. By the 1990s, defenses could no longer afford to predictably line up in eight-man fronts like the 46. In response, teams — most notably Tony Dungy’s Tampa Bay Buccaneers — developed an approach that took away the short throws to the flats and deep passes that had given the eight-man front so many problems. They did this by using a two-deep coverage look while sending the middle linebacker deep down the center of the field to take away passes to the tight end. Thus, football pundits — particularly of the television variety — had a new catchphrase: The “Tampa 2.” In this defense, safeties played deep to stop the big play.

But the Tampa 2, with two or three deep defenders, was weak against inside runs, and by the early 2000s offenses were too good to let you so obviously declare your intention: stop the pass or stop the run. Even teams that switched between eight-man fronts and two-deep coverages had trouble, since offenses could simply read the defense and shift between spread schemes and power formations. Defenses needed something better. They needed Ed Reed and Troy Polamalu.

Reed and Polamalu were part of a new breed of safety who could do everything a defensive coordinator needed. They could be Doug Plank in the 46 on one play and a deep Tampa 2 defender on the next. Defenses knew how to play pass coverage, but they couldn’t figure out how to do it while also holding strong against the run. The answer — made possible by these game-changing safeties — came in the form of “Cover 4,” also known as “Quarters.” It’s the most important defensive scheme of the past decade.

At first glance, Cover 4 looks like an anti-pass “prevent” formation, with four secondary defenders playing deep. But therein lies its magic. The four defenders are actually playing a matchup zone concept, in which the safety reads the tight end or inside receiver. If an offensive player lined up inside releases on a short pass route or doesn’t release into the route, the safety can help double-team the outside receiver. If the inside receiver breaks straight downfield, it becomes more like man coverage. This variance keeps quarterbacks guessing and prevents defenses from being exploited by common pass plays like four verticals, which killed eight-man fronts. The real key to Cover 4, however, is that against the run both safeties become rush defenders (remember, the outside cornerbacks play deep). This allows defenses to play nine men in the box against the run — a hat-tip to the 46’s overwhelming force.

This last point is crucial to why Polamalu and Reed thrive in Cover 4. Both safeties have responsibilities against the run; which role they take depends on which way the ball goes. If the running back rushes toward them, they usually have “force” responsibility — that is, they try to tackle the ball carrier. If the running back rushes away from them, they’re responsible for covering the cutback and bootleg. The interior defenders are typically given “gap” responsibilities to plug the holes they expect to encounter against a specific run play. Those inside defenders typically use something called a “spill” technique, meaning they chase the running back toward the sideline, where he can be cornered and gang-tackled. But if you’re going to force a runner to the outside, you better have someone waiting for him there. That player is the “force” player, and in Cover 4 that player is typically the safety, who has read the run and is in position to crush the ball carrier. Or at the least force him back into the linemen’s arms.

The backside safety’s job is even more important. He, too, will drop down for the run, but will do so cautiously as he looks to defend against a cutback. Many of the biggest runs in football come not in the initial burst but instead from the runner cutting back in the opposite direction, and it’s that backside safety’s job to be a sure tackler in space, something Reed and Polamalu do very well. With the safeties in support, the rest of the defenders can swarm the ball carrier. Of course, Cover 4 is but one tactic defensive coordinators can use. Pittsburgh, for example, mixes up its looks by sending Polamalu on a timely zone blitz straight up the middle to either stuff the run or bumrush the quarterback. Such variety is crucial, but Cover 4 has become one of the most important tactics for modern NFL defenses.

The history of football is essentially the history of ideas meeting talent meeting a moment. Decades of strategic tug-of-war preceded Reed’s and Polamalu’s careers, and they arrived at a point in the game’s development when their skills were particularly needed. Their versatility in defending both pass and run plays allowed NFL defenses to claim victory in one of these strategic battles. Reed and Polamalu have had the good fortune of playing for excellent coaches, but they’re also both so talented that they have bent coaches’ schemes to their strengths and ruined opponents’ carefully designed game plans. This is the beauty of watching these future Hall of Famers play: In every interception, in every tackle for loss, in every big hit and big return, football history is not only made, but also extended. Their brilliance on the field will continue to inspire the film room schemers to innovate, and football history will continue to be pushed by the twin forces of ideas and athleticism.

Chris Brown runs the website Smart Football. Follow him on Twitter: @smartfootball.

Previously from Chris Brown:

The Strategic Legacy of Al Davis

Darren Sproles and the Rise of the NFL “space player”

Football Strategy 101: West Virginia vs. LSU

NFL Strategy: Beating the Blitz

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