Take a look at the NBA’s defensive efficiency rankings, and you’ll notice the Boston Celtics at the very top, allowing just 95.7 points per 100 possessions. That number isn’t merely good; it would be the best defensive rating since the 2011-12 season, when Tom Thibodeau’s Chicago Bulls stonewalled everyone in their path. With their elite defense compensating for an inconsistent attack, the Celtics have bounced back from an 0-2 start to rattle off 13 straight wins.

None of this makes sense.

Over the summer, the Celtics replaced perimeter stoppers Avery Bradley and Jae Crowder with youngsters Jayson Tatum and Jaylen Brown, aged 19 and 21, respectively. And although Boston has defensive stalwarts like Al Horford, Marcus Smart and Terry Rozier, those guys were all on last year’s squad—which hemorrhaged 105.5 points per 100 possessions, a mark that placed outside the top 10.

So what’s causing this improvement?

Better rebounding is part of it. The Celtics are pulling down 81.3 percent of opponent’s misses this season—fourth in the Association—compared to 75.3 percent last year. Aron Baynes is immovable, and Horford is crashing the boards with newfound ferocity.

Luck is another part of it. Opponents have hit just 35.7 percent of wide-open threes (defender six-plus feet away) and an abysmal 29.8 percent of open threes (defender four to six feet away) against Boston. Only once since 2013-14 has a team’s opponents gone the full year shooting under 30 percent on open treys (2014-15 Houston Rockets).

But another reason for Boston’s stingy defense might just be Kyrie Irving.

To this point in his career, Irving’s defense has fallen somewhere between bad and catastrophic. Last year may have been his worst showing yet; he ranked 478^th out of 486 players in NBA Math’s defensive points saved metric, struggling in nearly every facet of defense, from closing out on spot-up shooters to navigating pick-and-rolls:

While defensive statistics are notoriously imprecise, the tape confirms the numbers in this case. Irving smashed into screens, botched rotations and gambled his way out of plays. Here, in Game 7 of the 2016 Finals, Irving tries for two wraparound steals he has no chance at nabbing and, in each instance, gifts an open three to the greatest shooter ever.

Exhibit A:

Exhibit B:

Forget elevator screens or hammer sets—in the last two Finals, the Golden State Warriors could run simple, stoppable plays for Stephen Curry and count on Irving to screw up. Yes, guarding a two-time MVP is difficult, but keep in mind that Irving has played some of his best defense against Golden State.

While many players are space cadets away from the ball, Irving has struggled on the rock, too. He’s been especially vulnerable against pick-and-roll sets, as you can gather from the chart above. Guards are supposed to stick to the ball-handler’s hip when defending a screen. Chris Paul and Avery Bradley are like Velcro in these situations. Irving has been more like a matador for most of his career, yielding driving lanes with little resistance:

The Kyrie Irving of past years couldn’t have played on a top-ranked defense. So, sure enough, Boston-era Kyrie has looked much more solid. And the biggest difference? Effort.

Irving has flashed above-average defensive ability in big moments but has never stayed engaged over the course of an entire season. This year, he’s fighting over screens, running back in transition and closing out with a purpose. And that effort is beginning to show up on the stat sheet. Irving’s 2.2 steals per 36 minutes are a career high, and he’s also averaging 3.6 deflections per 36 minutes, compared to 2.6 last season.

Steals can be misleading—Russell Westbrook is a serial gambler whose steal-hunting can expose his teammates—but Irving is forcing turnovers because he’s in the right position. Those live-ball giveaways result in points on the other end more often than not:

Irving still isn’t a plus defender against the pick-and-roll. Though his activity level is high, his technique navigating screens and forcing ball-handlers to the sideline still lacks refinement. But the Celtics don’t need Irving to be perfect; he’s intelligent and athletic enough to hold up with the right amount of effort. That diligence is finally a consistent part of his repertoire.

Boston head coach Brad Stevens has also worked to minimize Irving’s responsibilities. He usually guards limited wings like Tony Snell and Andre Roberson, often allowing him to hide in the corner instead of defending primary ball screens. The Celtics’ young wing pairing has, somehow, picked up the slack.

Not only is Kyrie more engaged defensively, but Stevens is putting him in much better defensive situations than Lue did.

— Brady Klopfer (@BradyKlopferNBA) November 7, 2017

Of course, Irving still has to chase shooters and nail rotations away from the ball, and, at times, he reverts to old habits. He has two or three brain farts each game—down from five or six in the past. He also helps from one pass away much too often—a no-no in a league so heavy on shooting:

But even off the ball, Irving has been worlds away from his old self. He’s proven comfortable and effective chasing guys through a forest of screens, while also realizing when to help or switch. In this play, he denies Goran Dragic the ball and seamlessly pivots after Dragic and Josh Richardson counter with a dangerous split screen:

That kind of switching has become a crucial element in Boston’s defensive scheme. As Charlotte Hornets head coach Steve Clifford said after playing the Celtics in early November, per MassLive’s Jay King: “They have size and versatility so they have a bunch of guys that can switch so they stay totally out of rotations as much as any team other than Golden State that I’ve seen.”

Boston couldn’t play this style with the 5’9” Isaiah Thomas. Bigger players would eat him alive after switches. Opponents actively targeted Thomas in many games, using his man as a screener to draw him into plays and create mismatches. Thomas isn’t a poor defender in terms of fundamentals. And he exerted reasonable effort on defense, given his burden on the other end.  But he’s still just 5’9.”

By contrast, Irving is 6’3” and more comfortable pestering wings. Opposing coaches don’t bother changing their entire offensive scheme to target him, especially now that he’s emerged as a decent defender. Irving has his warts, and he’ll likely never be a major plus defensively. He’s definitely not the main reason why the Celtics are shutting down their opponents. But he’s not a weak link either, so they don’t have to bend their defense to accommodate him.

The question of whether Irving is good, decent or passable on defense is just semantics. He’s much better than he was before.

All that leap took was a change of scenery—and a tiny bit of effort.

Follow Nate on Twitter @NateWolfNBA.

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Unless otherwise indicated, all stats are from NBA Math, Basketball Reference or NBA.com and are accurate heading into games on Nov. 15.