Fifteen games in, the story of the 2012 playoffs so far has been third-period comebacks. Twelve out of 15 games have featured a one goal lead at some point in the third period, and in seven of those twelve games the trailing team fought back to tie. Five times the team that was trailing eventually ended up winning the game, four times in overtime and once in regulation (the Flyers' roller coaster 8-5 win over the Penguins on Friday night).
I would have guessed that the trailing teams were riding very high percentages to be able to put up those kind of results, but that's not actually the case. Both the leading and trailing teams so far have shot 8.0% in the third period with one team up a goal, although the leading teams' numbers are slightly inflated because of empty netters (the shooting percentage with a goalie in the net for teams up by one is 6.1%). The primary reason for all the comebacks is that the trailing teams have been absolutely dominant in terms of possession, outshooting the opposition 88-49. That is a rate of 39-22 per 60 minutes of play.
It is normal for teams to play to the score. According to Behind the Net's stats, the Nashville Predators were the only team in the league this season that did not have over 50% of the Fenwick events while trailing by a goal, and teams down by one had an outshooting advantage of 29.6 to 24.5 per 60 minutes. Those numbers include results from the whole game, and would likely show a greater split for third period results only (some of my past findings indicate that the shot rate skews even more in the third period alone).
Despite generating more shots, however, the trailing teams were actually outscored during the regular season (2.54 to 2.43 per 60 minutes), which indicates that playing to the score was the right move for teams holding a late lead. It probably should be noted though that in a regular season sample team strength might also be a factor, given that the better teams would be more likely to spend time in the lead than the league doormats, whereas the playoffs do not have the same broad range of team quality.
The regular season results give a trailing team shooting percentage of 8.2%, very close to the 2012 playoff numbers. During the regular season the numbers for the leading team went up to 10.4%, which indicates that the comeback teams in this year's playoffs have probably been a bit lucky or had their comebacks enabled by strong goaltending in the third period. That said, even if the leading teams' percentages rise as the playoffs go on, there will still be a lot of comebacks as long as the shot ratio remains in the vicinity of 2-to-1 in favour of the trailing teams.
Utilizing a collapsing defence to preserve a late lead is the best strategy in some cases, but some teams (Pittsburgh and Phoenix, perhaps?) are probably currently reviewing their game tape to determine whether their late-game play is doing more to help them than to hurt them. You can trade quantity for quality in terms of chances against, but if the quantity gets too high than some pucks are going to go in anyway and the strategy is no longer optimal. It will be interesting to see whether teams can improve their ability to hang on to late leads or whether the comebacks will continue in 2012.