The Boston Celtics have made a leap this season, as their 13-9 record and +4.8 point differential reflects. They owe that entire improvement to one end of the floor.

They haven't taken another step by upping their scoring in Brad Stevens' high-paced offense. Instead, they've reached another level by locking down on defense. After finishing in the middle of the pack on that end last season, the Celtics currently allow the third-fewest amount of points per 100 possessions in the league. Only the Spurs and Heat have been better, and Miami has done so thanks to an easy schedule and a ton of home games.

It's not like the Celtics have amazing defensive personnel. The only additions to the team were David Lee, a defensive sieve, and Amir Johnson, a good all-around player that Toronto felt was past his prime. Marcus Smart, arguably Boston's top perimeter defender, has also been injured for most of the season.

How do the Celtics suddenly have such a great defense?

Boston shuts down pick-and-rolls without surrendering open threes

Defending the pick-and-roll well is a must for any good defensive team. The safe way to do it is to have the big man drop back to guard the play with two people instead of trapping and causing a third player to help. The Celtics follow this trend, but for it to work, every player needs to be working together in unison.

Brad Stevens has gotten his players to do just that. The Celtics are containing both threats on the pick-and-roll, ranking ninth in the league in points per possession allowed to the ball handler and 10th in points per possession allowed to roll men, per Synergy Sports. The guards always work hard to get back to the original assignment, and if they can't, Boston will switch to prevent an open shot.

They still commit too many fouls, but in general, Boston does well to contain those threats without compromising their three-point defense. They don't force a third player to be heavily involved unless it's absolutely necessary.

Opponents take just 21 three-point attempts per 48 minutes against the Celtics, the fifth-fewest in the league. They also connect on 33.3 percent of them, the eighth-lowest mark in the league. Teams generally have more control over the number of threes opponents attempt than the percentage they make, so Boston's combination can be sustained over the long haul.

That combination of solid pick-and-roll and three-point defense makes Boston hard to defeat in half-court situations.

The Celtics don't get beat in transition

Teams that crash the offensive glass or play at a high pace often have poor floor balance and don't make that extra effort to run back. Usually, a coach must choose between going for offensive boards or stopping fast breaks.

That's not a choice Stevens' club must make. The Celtics rank fifth in the league both in pace and in offensive rebound percentage, but still allow the second-fewest fast break points in the league. Even adjusting for pace, they are in the bottom five. Boston's opponents have only bested their fast-break point average four times in 22 games, and two of those (the Rockets and 76ers) did most of their damage in blowouts when the score wasn't in question. It's really hard to beat Boston down the court unless it's on a live-ball turnover.

How the Celtics do it is somewhat counterintuitive. Rather than having everyone rush back, they ask guards to lurk in the backcourt to slow down the ball handler or deny the outlet pass after misses.

That little bit of pressure forces opponents to use more of the shot clock to set plays up. Most teams have taken a larger percentage of late (7-4 seconds left on the shot clock) and very late (under four seconds on the shot clock) against the Celtics than they have on average, per SportVU data. The Celtics don't let opponents get any easy points, which is a big part of their success as a defense.

They are great at creating turnovers

The Celtics have a smart system and fundamentals in place, but they also several excellent perimeter defenders that are disruptive enough to create turnovers. Boston has three players in the top 10 in total steals -- Jae Crowder, Isaiah Thomas and Avery Bradley -- and lead the league in steals per game and opponent turnovers as a team.

The best part is that the Celtics don't have to gamble to generate those steals. Their wings have good hands and get in passing lanes but their guards in particular can strip opponents on the ball, which is not a common skill. They don't mind picking up ball handlers full court and applying pressure.

The steals fuel the Celtics' fast break and mask their poor half-court offense. They also provide a disruptive edge to a defense that lacks an imposing inside presence. The perimeter defenders set the example and the big men, who are not great shot blockers, follow their lead. All four of Boston's main rotation big men average at least a steal per 36 minutes. That's very rare.

On most good defenses, the big men set the agenda for the guards. The Celtics, on the other hand, flip that script. The ball is never safe against the Celtics.

The Celtics' defense is specifically geared toward stopping efficient offenses

Typically, the best offenses thrive in two areas: in transition and in pick-and-roll situations, either to directly score or as a vehicle to set up three-pointers. The Celtics are good at neutralizing those threats, or at least limiting the amount of damage opponents inflict in transition and from beyond the arc.

They will have a chance to test their ability against the best on Friday when they face the undefeated Warriors. A good showing against the defending champions proves that the Celtics' defense is not just a result of a relatively easy schedule. Instead, it's a result of a team that's better than the sum of its parts because of discipline and teamwork.