Matt Birk is a bright guy. Back in the day, he was an academic All-State honoree at Cretin-Derham Hall. As television commentators used to remind us constantly, he went to Harvard, graduating with an economics degree. Before retiring, Birk was regarded as one of the smartest players in the NFL.

But during a long pro career, few cerebral challenges frustrated Birk more than an early-season loss against the Cincinnati Bengals in 2010, when as the Ravens center he was tasked with trying to solve the Rubik’s Cube that was Mike Zimmer’s third-down blitz package.

“We didn’t have the right answer, so they just kept running the same thing over and over,” Birk said.

It all started with the two linebackers that the aggressive Bengals defensive coordinator walked up into what are called the A gaps — one lining up over Birk’s left shoulder, the other over his right.

Four years later, Zimmer is in his first season as Vikings head coach. From Cincinnati he brought a similar playbook and the same attacking mentality that helped him quickly turn the Bengals into one of the NFL’s stingiest defenses, something he seems well on his way to be doing here, too.

The signature scheme that Zimmer is known for — which is also the piece of intellectual property that Zimmer guards the most — are the double-A-gap blitzes that he calls whenever his defense can get the opposing offense into third and long.

Whenever those vulnerable situations come around, centers like Birk will look across the line of scrimmage and count seven Vikings defenders on the line inside the tackle box, all of them waiting to trample the quarterback.

The two linebackers in the A gaps might not be the ones who do it. In fact, they might not even blitz at all. But their presence can create a quandary for even the smartest center.

“Pass protection, it starts in the A gaps,” Birk said. “That’s obviously the shortest distance to the quarterback. Everything you do starts with, ‘OK, how are we handling A-gap pressure?’ ”

Not-so-secret secret

And for Zimmer, it started in Dallas, where he spent a dozen years as a defensive coach, then a coordinator.

About a decade ago, Zimmer and his fellow Cowboys coaches were chatting about different ways to affect offenses, and one idea was to put two linebackers in the A gaps. The details of when this conversation took place are vague because Zimmer doesn’t want to talk to a nosy reporter about it.

He is mum in part because he bristles at the notion of being an innovator, but also because he treats his blitz package as if he has the secret recipe for Coca-Cola.

“I really don’t want to spill the beans, you know,” Zimmer said.

Zimmer isn’t the first coach to put two linebackers close enough to the center that he could count their nose hairs. “Teams have been pressuring the A gaps since they started football and they’re never going to stop,” Birk said.

But Zimmer and the late Jim Johnson, who went blitz for blitz with Zimmer as defensive coordinator for the Philadelphia Eagles, helped take the scheme mainstream.

Now, “mugging” or “sugaring” the A gaps as it is called in coachspeak is something that most, if not all, teams have in their repertoire — including Sunday’s opponent, the Miami Dolphins, whose defensive is coordinated by Zimmer disciple Kevin Coyle.

Heck, even the Tampa Two Vikings under Leslie Frazier and Alan Williams did it occasionally.

After all, the NFL is a copycat league.

Zimmer is both flattered and frustrated by the fact that A gaps are mugged across the league.

“I’d prefer that [opponents] saw it [only] when we played them and not everybody else,” Zimmer said. “[But] I don’t know that everybody is as advanced. We’re kind of in the master’s program with that.”

Confusion is the key

NFL playbooks are no longer printed on paper, then bound together. Instead, they are now uploaded onto iPads. It’s hard for outside linebacker Chad Greenway to say exactly how large of a chunk Zimmer’s advanced blitz packages account for, but reasonable estimates are in the gigabytes.

“We’ve never seen the end of that thing,” Greenway said.

Greenway is one of the two nickel linebackers who is often in one of the A gaps as part of Zimmer’s preferred third-and-long look. The other was rookie outside linebacker Anthony Barr, but he is now out for the season, leaving Gerald Hodges to mug alongside Greenway.

Next to each of those linebackers is a defensive tackle lined up on the outside shoulder of a guard. Alongside them are the defensive ends, threatening to rush off the edges. And then there is an added twist that Zimmer throws in. He likes to bring one of his safeties, usually Harrison Smith, down to one edge, too.

But that doesn’t mean that they are all blitzing. That is why Zimmer’s vast array of third-and-long plays can leave offensive coordinators and quarterbacks guessing.

Sometimes one or both of the linebackers will back out, as will the safety. Or, if Zimmer really wants to make heads spin, one of the defensive linemen will drop into coverage in a zone-blitz look.

“They just don’t know who’s going to come and who’s going to drop,” said Smith, who leads all NFL defensive backs with three sacks. “We give them so many looks, just trying to confuse their pass protection, confuse the quarterback as well. And hopefully you free somebody up.”

In the Week 14 victory over the New York Jets, for example, the Vikings lined up with Greenway and Hodges in the A gaps on nine plays, seven of which were third and long.

On four plays, the Vikings rushed six defenders or more.

But twice they sent only four rushers and thrice they sent five, leaving offensive linemen blocking no one.

“Obviously, the offense doesn’t know that [they are dropping], but it’s not a guessing game,” said Greg Cosell, a senior producer and tape junkie for NFL Films. “You have to protect your quarterback.”

Offense battles back

And that brings us back to Birk, who claims the Ravens were finally able to crack Zimmer’s code, but not in that frustrating 15-10 loss in Week 2 over the 2010 season. Often sending multiple pass rushers off one edge, the Bengals hit Ravens quarterback Joe Flacco several times and pressured him into four interceptions.

“We just went with our rules and we weren’t able to adjust,” Birk said.

There are a few common ways that offenses try to deal with double-A-gap pressure.

One option is gap protection, where they have their linemen squeeze toward the center to help him deal with the two potential blitzers in the A gaps. But that could force a pass-protecting running back to try to block a defensive end 1-on-1.

Another option is slide protection, where all of the linemen step in one direction and block, say, the man to their right. But this can also leave the running back alone with an edge rusher.

An additional alternative is man blocking, but that puts the back in a still-unfavorable situation of having to block one of the linebackers streaking through an A gap.

“There’s a lot of different ways to do it,” Birk said. “It’s not just the center and guards. It’s the tackles. It’s the running backs.”

The next time Birk’s Ravens played Zimmer’s Bengals, in the 2010 season finale, they had a solution. They realized that the Bengals linebackers were reading Birk. If he stepped left, that linebacker dropped into coverage and the linebacker to his right attacked the other A gap.

So when the Bengals mugged the A gaps in that 13-7 Ravens victory, Birk would point before the snap at the linebacker he wasn’t going to block, which was the opposite of what he did when the Bengals were in any other defensive alignment.

The linebacker he pointed at would drop back, allowing Birk to take on the other A-gap blitzer, which freed up the running back to slip out into a route.

“The next time we played them, we were ready,” Birk boasted.

“Zimmer was the coordinator, so it was really him that exposed our weakness to it, our lack of attention to it, and forced us to become really good at dealing with the A-gap pressure.

“The reason we got real good at [dealing with double-A-gap pressure] is that Cincinnati was the team that killed us with it.”

Birk’s Ravens were the ones frustrating Zimmer that day, but the coach’s Rubik’s Cube is ever evolving, presenting different challenges for pass protectors seemingly every week.