Zach Lowe's 10 NBA things: The Celtics' incredible surge, a quiet Kyrie Irving development and elite acting by two Kentucky Wildcats

The Boston Celtics' ascent (finally!) has been fueled by the stingiest of defenses. Winners of 15 of their last 20 games, the Celtics' starting five has allowed a preposterous 88.8 points per 100 possessions. AP Photo/Michael Dwyer

With the first half of the NBA season coming to a close, this week we highlight a Time Lord-fueled Celtics ascent, leakiness from the Knicks, a quiet Kyrie Irving development to monitor and Oscar-level acting from two NBA All-Stars.

1. The Boston Celtics are, finally, who we thought they were

The most interesting thing in NBA defense over the last 20 games has been Ime Udoka's semi-radical decision to slot his shot-devouring center -- Robert Williams III, Lord of Time -- on wings away from the ball. The next-biggest Celtic -- Al Horford in Boston's starting five, sometimes Grant Williams, Jayson Tatum or Jaylen Brown in smaller looks -- takes the other team's main screen-setter, and switches everything.

The idea is to build a forcefield around the paint by switching up top, with Robert Williams looming along the baseline ready to pounce. A happy side effect is sewing confusion in offenses: Wait, where's Time Lord? Oh, there. So who's guarding our main screener? Are they just going to switch? Should we run our normal stuff anyway? Or divert our offense to attack Williams? But that would mean using a less dangerous screener, and Williams is really good at switching too! Oh, crap, there's 5 on the shot clock and Marcus Smart is six inches from my face.

Boston is about to overtake the Golden State Warriors for No. 1 in defensive efficiency. Their starting five has allowed a bonkers 88.8 points per 100 possessions -- easily the stingiest mark among lineups that have logged 100-plus minutes. Luck has helped; opponents have hit 29% on 3s against that group, and 34% against Boston overall. During Boston's current 9-1 stretch, opponents have shot 32% on midrangers. For the season, no team's opponents have underperformed their expected effective field goal percentage by a larger margin than Boston's, per Second Spectrum.

But Boston is driving this. Only the Warriors allow fewer shots at the rim. The Celtics have kicked their fouling habit. They are long and tenacious -- neck-and-neck with the weirdo Toronto Raptors as the best at unnerving shooters with flying closeouts. Opponents have made just 51% of shots at the rim with Williams nearby -- eighth lowest among 100-plus rotation guys who challenge at least three such shots per game. (One of the seven players above Williams is new Celtic Derrick White, who by most advanced metrics ranks among the league's 20 best defenders.)

Smaller groups with Time Lord as the only traditional big have been impenetrable; Boston's potential new closing lineup -- Smart, White, Brown, Tatum, Robert Williams -- might be a problem.

Opponents will concoct ways to attack Boston's unconventional scheme. (Brian Scalabrine and I brainstormed some on the Lowe Post podcast.) Boston will adjust.

In preseason, I labeled Boston a lock for a top-six spot -- with a chance at seizing No. 3. (Remember when everyone assumed Milwaukee and Brooklyn would go 1-2? Whoops.) That looked foolish for 30 games, but the optimism was about this defense.

The Celtics now have the East's best point differential. They are a threat to beat any conference rival in the playoffs, though a long shot against the Bucks. How the seeding shakes out will be pivotal.

2. The Miami Heat, coming into focus