The Giants' defense greatly improved in the playoffs. (Ted Kerwin/Creative Commons)

How did this happen? They could not stop the Saints two months ago as they fell flat on their faces. They lost to the Redskins twice: once in the beginning of the year, the other time, near the end.

They allowed more points than they scored in the regular season. Now, the New York Giants and their defense are sitting atop the football world.

How did this happen?

The remarkable turnaround of the Giants defense coupled with Eli Manning's "clutchness" (clutchitosity? clutchitude?) drove them through the post-season past three of the NFL's best teams.

Manning stories will appear as much as those crappy, "documentary-style" horror flicks, but the impressive thing is that once again, this defense banded together and played like they had nothing to lose with the ultimate prize at stake: a permanent place in NFL history.

The team followed the same strategy it for the past 30 years: start with the four men on the line of scrimmage and pass rushers, and then go from there. From Lawrence Taylor to Michael Strahan to Jason Pierre-Paul, elite pass rushing has been a staple since Bill Belichick dialed up blitzes for the G-Men back in the 80s.

But this goes back to before the playoffs started. Head back to Week 15.

The Giants let Rex Grossman and the 'Skins embarrass them. Sexy Rex was only sacked once, and though the team came up with two interceptions, they could not completely shut down the legendarily awful Grossman.

The following week was an affair against their stadium-mate: the New York Jets. The defense got back on track with its effectively aggressive tendencies, forcing three turnovers and knocking pretty boy Mark Sanchez to the ground five times. All-around, the defense halted the Jets attack. But what was really impressive was the next week.

New York was in win-or-go-home mode after the Washington debacle, but the Week 17 showdown with their bitter rival – the Dallas Cowboys – was literally a win-or-go-home situation: The winner takes the NFC East, and the loser has the opportunity to sit on their couches.

Osi Umenyiora was second on the team with 9.0 sacks in 2011. (Matthew D. Britt/Creative Commons)

Big Blue sprinted out of the gates, immediately turning the game into a track meet. Poor Tony Romo had the distinct dishonor of being crushed by the Giants front seven for six sacks, including two by Osi Umenyiora. Even Ben Roethlisberger thought the hits on the quarterback were excessive. The playoff berth was theirs.

Coming up the following week was the Atlanta Falcons, a team with a good offense. Not only were the line and quarterback good, but Atlanta featured playmakers that could test the secondary and run defense of the Giants. No big deal. Ain't nuthin' but a 'G-men' thang.

Michael Turner was burned by the swarming Giants tackling and coach Mike Smith's susceptible play-calling, as the only Falcon points that occurred were via a safety of Eli Manning. An easy win sent them the Giants to Lambeau Field to take on the mighty Packers.

But no, surely this team could not go on the road and take down the best quarterback and the best team of the regular season. Their defense couldn’t be that good.

Well, um... that's exactly what happened.

Either an alien travelled to Earth and took Aaron Rodgers' talent (à la Space Jam) in what was the worst performance of his spectacular season, or the Giants schemed against Rodgers so well, it threw him off his game. The latter is probably more likely.

The secondary played as well as it had all year. The coverage was so incredibly tight that Rodgers' knack to throw successfully into coverage was limited. On each side of the ball, the Giants were excellent recovering fumbles – they recovered three against the Packers. After the surprising win, the defense overcame its largest task of the season.

Alex Smith and the 49ers would be a much easier task on paper. But in execution? Not at all. Each team traded blows the whole time. They struggled covering tight end Vernon Davis, who had each of San Francisco's touchdowns. But the defensive line appeared to rattle Smith, who reverted to a performance that reminded people that he has yet to shake the draft bust demons he appeared to elude the previous week.

Ultimately, the San Francisco special teams saved the day with two critical errors that New York capitalized on. All that was left was the Super Bowl rematch.

The crew that terrorized Tom Brady was deeper than Biggie's at Bad Boy. Pierre-Paul broke out this season and will be strike fear into opposing quarterbacks for years to come – he just completed his second season. A physical specimen reminiscent of "The Freak" Jevon Kearse, defensive coordinator Perry Fewell moved Pierre-Paul around the line, even lining him up as defensive tackle sometimes. He deflected two Brady passes, one of which would have assuredly added seven for the Patriots. But he wasn't even the best Giant on defense on Sunday.

Giants DE Justin Tuck got the pressure on Brady. (Chris Pusateri/Creative Commons)

Four years ago, a then-unknown third-year Notre Dame alum named Justin Tuck had his coming out party, tearing down Brady two out of the five times he was sacked. He had all two sacks on Brady this time around, even saying his Sunday performance was "the best I've ever played." Tuck abused the Patriots offensive line.

On the first snap for the Patriots, Tuck tore through the line and past All-Pro guard Logan Mankins to get a lick on Brady. Brady threw an errant pass downfield with no receiver in the vicinity, drawing an intentional grounding penalty. Since Brady was in the end zone, it was a safety. Two points and Giants ball.

That play set the tone for the rest of the game where Brady was hit multiple times, including one that appeared to aggravate his left shoulder. After that hit by Tuck, Brady was 8-for-19 passing and unable to assemble another scoring drive to notch a Pats victory.

The popular theme of the 2011 post-lockout season was that, more than ever, the NFL was a quarterback-driven league. A team needs an elite QB to compete, and the days of Trent Dilfer fronting a team are gone. But right after the quarterback position, a high-quality pass rush is a close second. Look at the decade's past winners. Each team had some ability to attack the opposing quarterback by either elite rushers or great blitz schemes.

If more teams do not try to copy the Giants' formula in stocking up pass rushers, it will be a complete and utter shock. Then again, who never wants to be the best?

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Reach Will by email, or follow him on Twitter.