Also, with three two-gap linemen responsible for a total of six gaps, the defense sometimes can get away with having only helmet-on-helmet numbers in the box.
"Usually you have to outnumber them in the box, but if you do, you're a little bit lighter in the secondary," Kelly said last month. "So if you have defensive linemen that can two-gap and make plays off of two-gapping, then I think you can play with one less guy in the box at times."
Most defenses like to say their No. 1 job is to stop the run, but the Eagles take it to pathological levels, which may seem odd considering how the passing game has come to dominate the NFL.
But the Eagles often are more inclined to see the run on first and second down because opponents are trying to slow down the game against Kelly's fastbreak offense. And they believe if they can make offenses one-dimensional, they'll have greater success teeing off on quarterbacks.
"It's plus or minus 12 possessions a game. So you're looking for how you can concentrate and focus on each possession and how do you win those possessions," Azzinaro said recently. "Most of the time you can win those possessions if you can control the run game."
To Azzinaro, there was a direct correlation to stopping the run and winning games last season.
"In 10 of those games, we controlled the run game and we were able to win," he said. "And seven we really weren't in control of the run game and we lost."
While not entirely true, the Eagles did win six games when they held opposing offenses under 3.4 yards a carry. For the most part, they didn't surrender an inordinate number of yards per rush in their losses, either. Overall, they finished fourth in the NFL with a 3.8-yard average last year.
But in their playoff loss to the Saints, the Eagles allowed 185 yards on the ground at 5.1 yards a clip. The game plan was slanted to stop Drew Brees and the passing game with Davis playing a lot of nickel, but the Eagles' line struggled against New Orleans' mammoth blockers.
Nose tackle Bennie Logan didn't play poorly, but he was 305 pounds last season and some wondered whether he was a big-enough plug in the middle. He added 10 pounds in the offseason, though, and defensive ends Cedric Thornton and Fletcher Cox are north of 300 pounds.
"To find NFL players that could perform at our tempo . . . I don't know where you find those guys," Azzinaro said, combating claims that his starting unit was slight. "I don't know if there's any defensive line in the country that has three guys that weigh 315 pounds."
There is only one 3-4 line that has three starters heavier than 315 pounds - Baltimore's. While Thornton and Cox are 309 and 301 pounds, respectively, the Eagles have the seventh-heaviest line out of 16 3-4 base defenses.
Size, of course, can be overrated. The Eagles drafted 333-pound nose tackle Beau Allen, but he showed in the preseason that he was more than just a big body. Azzinaro may obsess about stopping the run, but he ultimately wants versatile linemen.
"I think when you look at 3-4 spacing, and you look at defensive linemen in that spacing, you're looking for a guy that can block protect, that can shed a blocker, that can tackle and rush the quarterback," Azzinaro said. "You can't cheat football. Ultimately, if you want to sustain success, you sustain success by having well-rounded guys."
And selfless guys, particularly up front. Defensive end Vinny Curry, who thrives as a penetrating pass rusher, admits it took him a while to learn that. The Eagles can roll out Curry on passing downs, but if he wanted to play more he had to become better at two-gapping.
"You either want to play or you don't play," Curry said. "That's what you've got to tell yourself if you want to be active. That's the only motivation I go off."
So while the pass rush may have suffered at the expense of stopping the run last season, at least in terms of the number of sacks, Azzinaro continues to preach the importance of team defense.
"If the correlation to sack numbers equaled Super Bowls," Azzinaro said, "then everybody would take their guys and put them in the widest . . ."
A reporter then interrupted Azzinaro to say that the Eagles tried something like that with the wide-nine scheme that predated the 3-4's arrival.
"I have no comment on that," he said.