The Bulls have been the league's best defensive team since Chicago hired Tom Thibodeau before the 2011 season. But they've achieved that in a sort of counterintuitive way. The NBA is a pick-and-roll league, and you can feel Thibodeau's stress building when he starts talking about facing "50 or 60 or 70" pick-and-rolls in the same game, "and with so many variations."
The pick-and-roll is a two-man play, generally involving a ball handler and a big guy setting a screen for that ball handler. Chicago has built its defense around an interesting principle: It wants one of the two guys directly involved in the pick-and-roll shooting at the end of it.
There are other rules and sub-rules,1 but Chicago under Thibodeau has consistently ranked at or near the top of the league in the percentage of opponent possessions that end with one of those two guys shooting, per Synergy Sports. Other teams, including the Heat, try to force one of the other three guys on the floor to beat them, but Thibodeau wants to make those other three guys borderline useless. That philosophy is based on a rather bold belief: The Bulls think their two defenders, with just a little bit of help, can beat your two offensive players and coax the exact kind of low-efficiency shot you don't want to take. "You're trying to get perfection out of it," Thibodeau tells Grantland, "trying to get as close to perfect as a team could possibly be."
Scary news for the rest of the league: The Bulls are pretty close. Watch film of Chicago's defense until your eyes bleed/your wife kills you — and I did — and the precision, so close to perfection, is overwhelming and almost beautiful.