The entire Miami Heat defensive gambit predicates on one thing, and one thing only: Chances. Extra chances are what spur their subsequent offensive assault -- only Denver and the Clippers scored more points off turnovers than Miami during the regular season.
Miami is still dangerous in the half-court, but especially so in transition, when the defense is scrambling to align properly and LeBron James can slide by unimpeded for a jarring dunks or Dwyane Wade can utilize the open creases in the defense to get easy buckets.
James made 429 shots inside five feet, many of which occurred in transition. That mark led the league. He attempted a bunch of shots there also; only Greg Monroe, Dwight Howard and Russell Westbrook were more frequent visitors to the most efficient spot on the floor.
Beating Miami means not giving them extra chances. For the most part, the Spurs have limited the Heat from these high-efficiency opportunities. Miami has scored 44 points off 34 San Antonio Spurs' turnovers in three games -- a rate nearly equivalent to the Charlotte Bobcats' regular season mark, which is nowhere near Miami's usual average (they scored 18.6 points off turnovers per game). It's not always easy to avoid these mistakes, especially when Miami is pressing their athletic advantage on every pick-and-roll, but the Spurs have consistently hit their safety valves and capitalized on poor backline rotations. Simple basketball. They've gotten into trouble occasionally by trying to make the "pretty" pass instead of finding the next open guy but that hasn't occurred too often.
While the Spurs have avoided deadly live-ball turnovers that Miami often turns into points, the Heat have coughed up their fare share of opportunities on the defensive boards. It's a combination of effort and a fundamental flaw in their "pace and space" infrastructure. The Heat were one of the worst rebounding teams in the league during the regular season, and that is in large part because coach Erik Spoelstra intentionally sacrifices length for shooting and offensive versatility. They allowed opponents to score 14.9 points per game off second chances, the second-worst figure in the NBA. San Antonio, normally apathetic on the offensive glass, has been even better than anyone could have reasonably expected in this regard. They've scored 56 points on second chances in three games, giving them a whopping 23 point advantage over the Heat.
Effort plays a huge role in this, too. The Spurs have played their fair share of small lineups to combat Miami. And they are still dominating the offensive glass. Wade has been lazy on his fair share of box-outs -- Tracy McGrady has more rebounds (5) than Wade (4) in just 14 minutes -- and James is not always focused on the boards though he's averaging 12.3 rebounds. Tim Duncan and Kawhi Leonard have been especially cognizant of the these deficiencies; they are responsible for 26 of San Antonio's 40 offensive rebounds. That's almost nine extra chances per game generated by just two players.
Basketball is heavily influenced by chance -- normally expressed in shooting percentages. You either make the shot or you don't; it's all up to probability. When the Spurs' offense gets extra chances, you essentially author your own obituary, because they are ruthlessly efficient. Miami will be one step closer to that point if they can't create more chances than they give up. Otherwise, they'll have their backs against the wall, facing an almost insurmountable 3-1 deficit, and San Antonio rarely fails to put the opposition out of their misery.