Gamers crave absolution.
For an array of reasons, we don't like to lose. Some of that comes from the "game" part of the hobby but that's not all of it. We most get into this because we have ideas of heroes triumphing over overwhelming odds through pluck, cunning and sheer force of will, and we don't want to come up short. We want to hold the line. We won't let it happen on our watch. We will run barefoot across broken glass in Nakatomi plaza because somebody has to.
This creates a lot of problems, especially when you're trying to do something that depends on players losing. I'm not talking about randomly losing a fight or pulling off a TPK, but rather the kind of defeats and setbacks that mark the second act of a lot of stories. Even the most trivial of these sorts of problems will often be met by players digging in their heels, putting up their fists, and refusing to let it happen.
At this point, things get ugly. Even if the GM is totally fighting fair, the players may be angry and resentful, and accusations are likely to be thrown about (or worse, linger unspoken). The easy fix for this is, of course, to skip the second act entirely, and that solution works - it's demonstrably worked just fine for gaming for decades - but sometimes you just want to push it a little farther.
This is where absolution comes in.
The trick is that defeat is not about the characters, it is about the players, and as a GM, you need to address those players. You need to get across that the tragedy is coming, whatever the characters do. It is not on their heads, so with that settled, are they willing to get onboard with making it awesome?
It's a simple proposition, but one that is not natural to all players, but usually one or two in a group can pick it up pretty easily, and they provide an example for the rest. An example of what? That's the trick: once players have bought into the idea that things are going to go bad, you need to give them the ability to really direct how badly it goes. Given that, players will generally be much, much nastier to themselves than you would EVER be willing to be.
It's one of those balancing acts of trust and control. Because they know the outcome, the trust level is high, but they still want to maintain control. They're willing to fail, but they need to be allowed to fail big. By absolving them of the normal responsibilities that come with failure, you free them to go for the gusto in ways they never can normally.
One of the best examples of this is in Exalted. It is a fact of the setting that the Solar Exalts of the the First Age fell, and the heroes of the game (by default) are the reincarnations of those dead heroes. Because they're tied to that past life, it is not uncommon for events from the first age to come up, most notably in how the character died back then. Players given leeway to describe how that went will, provided they've bought into the idea, make sure their own falls are magnificent, enthusiastically throwing themselves into their character's death because they accept it as inevitable. There is no way for them to be so cunning or charming or stubborn that it won't happen, and with that relation, they seize on the one thing they can do, which is to make it awesome.
1 - This is, incidentally, one of the hardest parts of trying to use fighting anime (and other fighting material) as source material for gaming. For all it's visual awesomeness, the default mode is that the hero gets beat to crap, but won't give up and fights harder at the end, and that's enough to win. Very dramatic, but it makes for crappy play.
2 - The closest you can come is announcing that this is the last session of a game. That can, at times, be intensely liberating.