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NEW YORK — The Seattle Seahawks take a basic pass coverage scheme and play it in an extraordinary way.

How often the Denver Broncos can break free of the Seahawk pass defenders figures to be the most critical matchup of Super Bowl XLVIII on Sunday.

Seattle allowed the fewest passing yards in the NFL partly because it has the best defensive secondary in the league – the self-proclaimed Legion of Boom.

Richard Sherman and Byron Maxwell form one of the best big cornerback duos in the NFL. Sherman is 6-foot-3, Maxwell 6-1. All-Pro Earl Thomas is the top free safety in the league. Kam Chancellor is a versatile strong safety who can tackle and cover underneath zones.

“They’re just a talented secondary,” said former NFL coach Steve Mariucci. “They get help from the pass rush. They don’t have to add a fifth and sixth guy very often to get pressure, so they’re covering with seven most of the time. That really helps them, that’s why they’re No. 1 against the pass.”

The Seahawks rely heavily on a Cover 3 zone defense, a common scheme used at every level of football. The two cornerbacks are responsible for covering the outside deep thirds of the field. The free safety covers the middle third. Three linebackers and the strong safety are divided among four underneath zones.

Unlike most teams in the Cover 3, Seattle often asks its cornerbacks to play press coverage at the line of scrimmage, which puts more pressure on them to cover more of the field. Typically the Cover 3 is vulnerable to short, quick throws because the cornerbacks are bailing out to guard the deep third.

“What Pete Carroll is doing, not to overstate it, but it’s revolutionizing the game a little bit, along the lines of the West Coast offense and the Tampa 2 defense,” said former NFL coach Brian Billick, referring to Seattle’s head coach. “Playing long, with the athletes that he has, it’s got the attention of the league.”

“You better be good to do that,” Mariucci said. “They have free safety help, but that’s not going to help on the sideline. The safety’s not going to get that far over. So they’re special at corner.”

A fifth-rounder in 2011, Sherman’s development pushed the Seahawks to adapt their Cover 3.

“We were looking for a guy who can do that,” said Seattle defensive coordinator Dan Quinn, referring to Carroll’s first season, 2010. “We didn’t play exactly in that fashion my first year here when we were built differently outside. We still played three deep, but we didn’t press as much.”

“You need the length and the strength to play at the line, as a corner,” Quinn said. “You have to have a certain amount of confidence to want to play at the line of scrimmage and get your hands on somebody.”

Opponents complain the Seahawks go too far in putting hands on receivers beyond the legal limit of 5 yards from the line of scrimmage.

“They have perfected holding,” said Falcons receiver Roddy White in December.

“They’re not going to change throughout the course of the game, they’re going to push the envelope,” said Matt Bowen, a former Bills safety and analyst for Bleacher Report. “They’re so physical. You can do that when you’re playing press coverage. If you’re playing off-man at 8 yards and you grab someone, it’s a flag. If you’re up at the line of scrimmage, it looks like a fight. That’s what they want. They want to create a fight at the line of scrimmage.”

“They’re going to force the refs to make them change, and I still don’t think the refs will do it,” Bowen said.

Seattle has allowed an NFL-low three pass plays of 40 or more yards and a league-best 30 pass plays of 20 or more yards.

“Earl Thomas is so good,” Bowen said. “With him in the deep middle, he can take away the seam and the post to the deep middle. So Peyton Manning trying to move Earl Thomas will be a great chess match.”

Every defense has weaknesses, however, and if any quarterback can exploit them it’s Denver’s Manning, who led the league in passing. Denver had the second most completions of 20-plus yards (68) and the sixth most of 40-plus yards (13).

Expect Denver to attack the Cover 3 with three or four vertical routes down the seams of the field and with flood routes, in which receivers are running on three different levels to one side of the field.

Denver typically exploits man-to-man coverage with bunch formations, lining up three receivers tightly on the same side of a formation. How well can Seattle disrupt Denver’s big receivers? Demaryius Thomas and Eric Decker both are 6-3, and tight end Julius Thomas is 6-5.

Denver likes to split Julius Thomas out wide by himself on one side of the field. That could force Chancellor or linebacker K.J. Wright into man coverage against him, the kind of matchup Manning typically exploits.

“If they’re going to press your receivers and you are in a timing offense, that messes you up a little bit,” said former Super Bowl quarterback Kurt Warner. “The other thing is pressure. This team gets a lot of pressure. No quarterback is the same with pressure on him.”

“Those are the biggest challenges for Denver,” Warner said. “Can they get off the bump? Can they get the ball out of their hands quick enough, make quick throws so Peyton doesn’t have to be under duress?”

email: mgaughan@buffnews.com