OWINGS MILLS, Md. -- The Baltimore Ravens are using colorful gamesmanship in trying to disguise their defensive signals from the sidelines.

Three assistant coaches -- each wearing a green, yellow or red hat -- relay the calls to the field. It's unknown whether all are sending in signals to their specific position group or if two are conveying "dummy" calls.

If that wasn't enough subterfuge, the Ravens' injured and practice squad players stand behind those assistants and hold towels over their heads to shield them from the opposing coaches' box behind them.

"It’s kind of one of those things we probably don't talk about," coach John Harbaugh said.

Stealing signals has long been part of the game, and it created headlines with Spygate in 2007. New England coach Bill Belichick was fined the NFL maximum $500,000 and the Patriots forfeited a first-round pick for videotaping an opponent's defensive signals.

Improved technology -- coordinators can communicate with one player via a helmet radio -- has decreased the stealing of signals. But there are instances when teams need to send in calls quickly and efficiently to everyone on the defense.

"When they run a no-huddle offense, they force you to operate more quickly sometimes and to make calls through signals and things like that," Harbaugh said. "And we would prefer that he doesn’t know what defense we’re in."

So, instead of one player conveying the call to the rest of his teammates, all the defensive players have to do is look at the assistant coach wearing the right colored hat. The Ravens first began disguising their signals for some games last season, and they've gone with more this season.

It's a system that the Ravens have used when going against the likes of Tom Brady, Peyton Manning and Andrew Luck. Baltimore also used this when playing Philip Rivers in its last game. The Ravens, whose defense ranks No. 25 in the NFL, needs to keep any advantage they can.

The other part of this "game within a game" is players holding towels to block the signals. There are two players who are assigned to stand behind each assistant coach.

It can be a more laborious job than you think.

"You get a little tired, especially late in games," said defensive end Brent Urban, who is on the injured reserve designated for return list after having surgery to repair his torn biceps. "But it's good rehab for me."

For good reason, the Ravens don't want to give away any of their secrets. When asked if only one of the three assistants is giving the correct signal, Harbaugh said, "Can’t give you any details on that. I’m not allowed. I do not have clearance to give any details on the specifics of that."

Harbaugh did confirm that the colored hats had to be specially made with the Ravens logo on it. But, after a couple of minutes on this subject, he politely excused himself.

"I’m getting uncomfortable," he said with a smile.