The Pittsburgh Steelers haven't exactly faced three dynamic offenses the past three games, including one with a backup quarterback. But the improvement by the defense is noticeable in that span.

At points this season, the Steelers had the look of a team that would win only when the explosive offense would outscore the opposition. They had issues on the back end of their defense, age was creeping in at some key spots and the outside pass rush was hit by injuries and a lack of production from top picks.

After getting steamrolled in Week 10 by the Cowboys in a 35-30 loss, the Steelers were ranked 25th in yards and tied for 14th in scoring. They are now 14th in yards and seventh in scoring.

A pass rush that was 31st in sacks per pass play is now 17th after getting 13 of their 25 sacks the past three weeks.

It is a defense that appears to be jelling at the right time, ready for a push to the postseason and a playoff run. Yes, they beat up on a bad Browns offense, and beat the Colts without Andrew Luck, but limiting the Giants to two touchdowns, one of the garbage-time variety, was impressive last week.

So what has changed?

I studied the Steelers tape and several things stand out as the reason for the improved play. Here are five noteworthy ones:

1. The rookies are growing up

The Steelers start three rookies: corner Artie Burns, safety Sean Davis and nose tackle Javon Hargrave. All have been instrumental in the improved play of the unit. Burns is a big corner who has developed in coverage as the season has gone along. Davis is a rangy safety who is more than willing to tackle, yet has the range to cover a lot of ground. Hargrave is a power player at an all-important position in their defense.

I wanted to focus on Davis and Hargrave, especially Davis. When the Steelers were good on defense in recent years, they had Troy Polamalu making plays all over the field. Davis is starting to play like that, although he's got a ways to go to get to that level.

Davis, who played corner at Maryland last year, can play in the deep middle or up near the line of scrimmage. His ability to diagnose and attack is good for a player with so few starts.

There was a play from last week's game against the Giants where he came from the deep middle in a Cover-3 look to attack the runner and make a play. It was also a play that highlighted Hargrave's value to the team, particularly against the run. That will be key this week against the Bills and LeSean McCoy.

Here's a look at that play from the Giants game.

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The play is designed to go inside, but Hargrave beat Giants center Weston Richburg and clogs that hole, which makes Paul Perkins hesitate and try to jump outside to his left.

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There looked to be room, but Davis came from the middle like a rocket and made an impressive tackle on Perkins, with Hargrave getting over to help.

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That's two young players showing off their ability, with Davis really impressing on the play.

Here's a play where Davis was lined up in the box and made a great play in the run game, showing his versatility to play both deep and in the box.

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On this play, Davis was lined up just outside of edge rusher Bud Dupree. But when Odell Beckham Jr. motioned inside to the wing, Davis stacked behind Dupree.

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At the snap, tight end Jerell Adams blocked down on Dupree with a good seal block, and that left Beckham to block Davis.

But Davis recognized the run was coming in his direction and knifed through before Beckham could get a block on him to drop Rashad Jennings for a 1-yard loss.

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That's a veteran type of play because if he hesitated a split second, Jennings might be able to pop outside and pick up good yardage.

Davis also had an interception last week in the deep middle on a bad throw by Manning.

While Hargrave is primarily a run stuffer, here's a look at a sack he got against the Colts that shows he has the potential for so much more.

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On this play, Hargrave was lined up outside of the left shoulder of left tackle Anthony Castonzo, which is not a position he plays a lot.

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The Steelers showed a double-A gap blitz, but only one came. That left Hargrave, who stunted inside, to be handled by a double team with center Jonotthan Harrison and left guard Jack Mewhort.

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Hargrave was able to split the double with his power, but then was able to use his athleticism to chase down Scott Tolzien and trip him up as he tried to leave the pocket.

2. James Harrison is playing like a kid again

After getting two sacks in the first nine games, Harrison has one in each of the last three games and was especially impressive last week against the Giants. In addition to his sack, he had several other pressures and lined up in a variety of spots that seemed to confuse the Giants at times.

Harrison isn't the player he once used to be, but in spurts he is more than capable of winning his share of battles against tackles and running backs -- when the latter groups gets the unenviable task of trying to handle him.

James Harrison has turned it up in the last three weeks. USATSI

That happened last week against the Giants when Jennings got caught in a situation where he was one-on-one with Harrison. He whiffed with Harrison dumping Eli Manning for a sack. With Dupree back playing again after missing time, and Anthony Chickillo improving to go with Jarvis Jones and Arthur Moats, the Steelers have a bunch of players they can use to attack from the edge.

None of them are in the Von Miller-Justin Houston-Khalil Mack class, but they have bodies that are capable of disrupting the passing game. They need Jones and Dupree to really get it going. But even if they don't, it brings me to the third reason for the Steelers' turnaround.

3. Coordinator Keith Butler is confusing QBs

Butler is doing some interesting things to create favorable matchups and cross up the opposing quarterbacks and centers who make the line calls.

Here's a look at Harrison's sack last week, one that shows Butler's creativity.

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The Steelers showed an overload front to the offensive right side with a double-A gap blitz look. That forced Manning to change the protection to slide in that direction.

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The twist was Harrison lined up behind the two inside linebackers. When Manning checked, he jumped outside to the offense's left. With tight end Will Tye out in the pattern, it left Harrison one-on-one with Jennings. He jumped over his cut-block attempt to sack Manning.

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There were many more exotic looks that seemed to confuse Manning and the Giants last week. Butler was good with his creativity. Harrison got one pressure from an inside spot when hovered near the line of scrimmage.

I like the way Butler has called games the past three weeks.

4. Stephon Tuitt has picked up his play

The Steelers defensive end has gotten after it with Cameron Heyward out for the year with a torn pec. The Steelers lost arguably their best defensive player for the season when Heyward went down. That meant they needed somebody to step up, and that has been Tuitt.

The past two weeks, he has been really impressive as one of the down players. He has been active pushing the pocket and been really good against the run. Tuitt needs to continue playing big as Pittsburgh's best down player.

5. Ryan Shazier and Lawrence Timmons are back

Shazier missed four games with an MCL sprain, the third consecutive season he has missed games because of injuries. When he's on the field, he's an explosive playmaker. Yes, he can overrun plays looking for the big hit, but his speed is a big part of the Pittsburgh defense. He can get to a lot of runs and he's usually good in coverage.

A healthy Ryan Shazier makes the Steelers defense go. USATSI

Timmons, who turned 30 in May, was once also an explosive playmaker. But age has slowed him some, and he struggled early this season. With Shazier back, he has been closer to what he used to be.

He looked fast again against the Giants, jumping around making plays in the run game and also doing a nice job in coverage, getting a big interception in the red zone to turn the game back in the Steelers favor as the Giants were going in to score. He's clearly happy to have his inside running mate back on the field.

This is the unit that will decide whether the Steelers are a Super Bowl team. We know the offense will get it going. There are too many weapons and they've done it before.

If this defense can continue to grow together, the Steelers might be the biggest challenge of all to the Patriots in the AFC.