The sample size of historical NBA finals matching the circumstances of this year's Heat-Spurs match-up is small. Still, let's take a look at similar series over the years. It should first be noted that the current 2-3-2 finals format (i.e., two games at the home of the team with the better regular-season record, the next three games at the opponent's home, and the final two back in the first city) began with the finals of the 1984-85 season. Here are the final series since then in which a team has won Game 5 (abbreviated "G5") at home to take a 3-2 lead, and then sought to win the title on the road (data from here).
|Year||Won G5 @ Home||Opponent||Series Outcome|
|1984-85||Lakers||Celtics||Lakers in 6|
|1987-88||Pistons||Lakers||Lakers in 7|
|1993-94||Knicks||Rockets||Rockets in 7|
|2005-06||Heat||Mavericks||Heat in 6|
|2009-10||Celtics||Lakers||Lakers in 7|
|2010-11||Mavericks||Heat||Mavericks in 6|
As seen in the table, it's a wash: Three teams (shown in red) used their home Game-5 victory as a springboard to close out the series on the road in six games, whereas another three Game-5 winners at home (shown in blue) never got the clinching fourth win.
Two Michael Jordan-era Chicago Bulls squads entered a road Game 6 with a 3-2 lead in the finals and won the title in six, both times. However, those two experiences -- vs. Phoenix in 1992-93 and vs. Utah in 1997-98 -- are different from those in the table above. In both cases, the Bulls had led 3-1 and failed to close out the series at home in five.
We can expand the database by looking at years prior to the switch to 2-3-2, in which a 2-2-1-1-1 format (usually) was used for the finals. Under this format, a team that won Game 5 at home to go ahead 3-2 had the opportunity to wrap up the series on the road in Game 6 (although the home team for Game 5 knew that it would also play Game 7 at home, if necessary). Staying within the past 50 years, there were eight series in which a team won Game 5 at home to take a 3-2 lead and then went on the road for Game 6 (but not Game 7). They are as follows.
|Year||Won G5 @ Home||Opponent||Won G6 on Road?|
Again, it's a wash. Four teams (shown in dark green) won Game 5 at home to take a 3-2 lead and then captured the title on the road in Game 6. However, another four similarly situated teams (shown in orange) failed to do so.
If we assume that a road team in basketball usually has less than a 50/50 probability of winning, then the fact that a team that won Game 5 at home to take a 3-2 lead won Game 6 on the road 50% of the time in the above years suggests that there may be something to the notion of momentum in this context. There's a (somewhat) complicating factor, however. With the 2-2-1-1-1 format (prior to 1984-85), the road team in Game 6 would have been the team with the better regular-season record and thus arguably the better team. That may be why Game-5 winners may have done well in Game-6 road contests, as much or more so than benefiting from momentum.
In the 2-3-2 era, in contrast, the road team in Game 6 would be the one with the worse regular-season record. Hence, a 50% success rate for such teams in Game 6 speaks well for momentum.