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If you watched the Philadelphia Eagles against the Carolina Panthers on Monday night, you glimpsed one of the oddest scenes you'll see on an NFL sideline.

A few times between plays when the Eagles were on offense, the camera cut to coach Chip Kelly pacing in front of five men frantically gesturing toward the field. They looked like five out-of-sync third base coaches.

The scene bordered on comical. But it was serious business and code work for the Eagles.

The men were distinguishable primarily by what they wore. Four of the five had bright red wrist bands. Three were dressed in green sweatshirts, two in black. Three had green hats, one had white and one had his hat backward.

After Kelly called the play into his headset, each man started signaling. Maybe his moves meant something, maybe nothing. It's all part of Kelly's efforts to run a turbo-speed, no-huddle offense while ensuring that opponents can't crack the code.

"They're the fastest in the league," said Dom Capers, the Green Bay Packers' defensive coordinator. "(Kelly's) philosophy is to run as many plays as he can possibly run, to stress the defense, to try to get you uneasy and get you out of your rhythm."

NFL offenses, even in no-huddle, don't signal in play calls because quarterbacks have a helmet speaker. The Packers, for instance, run a predominantly no-huddle scheme where coach Mike McCarthy tells quarterback Aaron Rodgers the play, and Rodgers relays the call to his teammates with voice commands, hand signals or both.

Kelly, though, is unique in the league because he still deploys the signaling system he used at the University of Oregon. He does it purely for speed.

The sideline signals mean his quarterback, who will be Mark Sanchez this week against the Packers, doesn't have to relay the call to his teammates. Instead, they look to the signalers for everything they need to know: formation, play call and snap count.

The number of plays a team runs per game isn't the only measure of tempo — how quickly it scores, its number of first downs and the quality of its defense also factor into play counts. But the Eagles rank No. 2 in the league in average plays per game (71.2) — behind only Indianapolis (74) — and is strong evidence that they're playing fast.

A review of the Eagles' game from Monday night shows that while they don't always operate at a consistent tempo, they regularly have as few as 14 to 16 seconds between the end of one play and the snap of the next.

"It's a rapid pace," Packers cornerback Tramon Williams said. "Last year I don't think they went at that fast a pace, I think they were going every 22-ish seconds they were getting a play off. That's pretty fast, too. But 15, 14 seconds, that's really fast."

Kelly's rapid, read-option system has produced one of the NFL's top offenses even though he doesn't have a premier quarterback.

Of the five top-scoring teams in the NFL, four are run by Indianapolis' Andrew Luck (first), Denver's Peyton Manning (second), New England's Tom Brady (third) and the Packers' Rodgers (fifth).

The Eagles rank No. 4 even though Nick Foles is tied for second in the NFL in interceptions (10) and ranks No. 25 in passer rating (81.4). He's out for an extended time because of a broken collarbone, but the offense has functioned about as well with Sanchez, who in 1 3/4 games in Foles' place has put up a 97.7 rating in wins over Houston and Carolina.

"What I respect is (Kelly) has come into this league and put his system in, and his system's been very productive," Capers said. "Obviously it's been quarterback friendly."

The sideline signaling is a huge part of the system, and good luck to any team trying to figure it out.

Kelly and the Eagles don't talk about it publicly, so its details are not yet common knowledge around the league. But it's a given that the system has two qualities that enable it to work as well as it does: one, that it's simple enough for players to process on the fly, and two, that opponents can't crack it.

In fact, an independent party took a shot at deciphering Kelly's code late in 2010, when Kelly was coach at Oregon. Late that season, ESPN.com hired a mathematics professor and decoding enthusiast from Whittier College, Mark Kozek, to study two Oregon games for a story leading up to the Ducks' national championship game against Auburn that season.

The study concentrated on the six or so placards Kelly has used at times as part of the signaling system. The placards are divided into quadrants, each with a picture or word that triggers an association for the players. Kelly used the placards last season, but none of the sideline shots from Monday night showed them, and Capers said the Eagles don't appear to have been using them of late.

Regardless, Kozek was unable to match placards with plays, likely for two reasons.

One, many of Kelly's calls appear to contain multiple options, including options where the play can be a pass or run, depending on how the quarterback reads the defense after the snap.

"At every step in the play, there are many different reads," Kozek said in a phone interview this week, "and the way they prepare for those is practice, practice, practice, practice, practice, practice, practice. Then as the game progresses, the defense will make more mistakes, which makes the reads easier."

Two, Kozek determined that a signaler on the sideline wearing brightly colored wristbands probably was the key indicator. Kozek made up an example that, for instance, if the signaler moved one hand it meant a standard formation with a family of plays, and if he moved the other then the players knew to look elsewhere immediately for the formation and family of plays.

There's also a suspicion that some of the signalers are relaying information to separate position groups, which would speed the play-calling dissemination. There also might be one signaler sending dummy calls.

Kozek likens it to the system that American troops used in some theaters in World War II and that was depicted in the movie "Windtalkers." The Americans' code was a Navajo soldier on each end of a radio speaking his native language. The communication was simple for them but impossible for enemies to decipher.

"They know exactly what they're saying, and they're not making it too complicated," Kozek said of the Navajo soldiers. "But no one else in the world can understand the Navajo language, so (code breakers) are just SOL. We think it's something like that. There are probably one or two or three very specific, very simple indicators."

Kelly's final level of protection is that he can change the indicator signaler at any time. So there are so many possible combinations with five signalers that it's not worth the time for an opponent to try to figure it out.

"I'm not even looking at it," Williams said. "I don't think you can pick up on that, five guys, unless you really have a source who tells you what's going on."

The end game is that the signaling helps the Eagles play faster than anyone else in the NFL. Kelly's hope is that the tempo will wear down and bother the defense more than his players, because his team practices at that pace every day.

On Sunday, every Packers defensive player will get the call from sideline signals, rather than from the more conventional source, A.J. Hawk, who has the defense's helmet speaker.

Last year when the teams played at Lambeau Field, the Eagles won 27-13 and put up 415 yards in total offense. It's worth noting that Seneca Wallace and Scott Tolzien were the Packers' quarterbacks that day because Rodgers was out with a broken collarbone. The Packers' defense also practices daily against McCarthy's no-huddle offense, though it doesn't work at the Eagles' pace.

"That helps a lot in terms of handling it," Capers said. "I didn't feel a year ago when we played them here that the fast pace was disruptive to us. We'd really planned and prepared for it. I'm hoping that's the case come Sunday."

— pdougher@pressgazettemedia.com and follow him on Twitter @PeteDougherty.

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