I think that in that case, they are trying for a number 2. I think it was said earlier that dying sends you up one level, unless you are under sedation in the level you're going to. Saito died on the first level, whose level above it(reality) had forced sedatives, so he ends up in Limbo (after Cobb gets out of it, but before Cobb re-enters it again by dying in the van by drowning, giving Saito time to reconstruct it.)
In Fischer's case, he had to end his 'sleep' (wasn't really dead) by dying in Limbo, and Ariadne had to get out by the same method, which they both do (falling off the building), at the same time as the defib. Then again, I'm not sure about the point of the snow fortress being destroyed in the first place. Unless Eames blew up the fortress just in case, using the defibrillator as a signal of how much time they had down in Limbo, to make sure they were out of that lower 'level' for the next kick (the elevator). He had only a rough estimate at the time of the kick, I think (headphones on Arthur's ears filtering down through two levels)
I may have to watch it again to confirm this, though.