It's not so much that it's a twist or even a surprise - the movie fully expects you to be playing along at all times - it's just a more enjoyable experience going into things with a completely blank slate. If you see Cabin in the Woods, you'll be watching a good movie, simple as that.
Especially in a particular scene toward the end of the movie.
Part of the fun of Cabin in the Woods, as mentioned before is how self-aware it is. It brings up all the tropes (in fact, the tropes themselves make up the basis for the plot) so it shouldn't be any surprise that it uses these tropes to deconstruct horror movies while at the same time using them to make a better horror movie.
Take, for example, the Deus Ex Machina.
They're almost always frustrating as hell. Something just sort of pops up out of nowhere to save the heroes, and it's not ever really explained in story. In the movie, Dana and Marty escape the titular cabin, only to find that everything is even more horrifying in the complex beneath. Strange creatures, nightmarish and otherworldly, are condoned off in glass containers. It quickly becomes apparent to the two just how much more horrible it is than they even knew.
Then the director starts talking...telling them of the full purpose of this place. They're human sacrifices (though if you were watching during the opening credits, you had a pretty good idea what was coming, already). The armed men with the guns come in, and Dana and Marty are trapped in a little control room.
Why exactly the "system purge" option exists is not something I spent much time thinking about - it's just one last self-aware look at horror movie tropes. No, the glory is what happens when the big, red button is pressed.
Have fun trying to deduce the different references to horror movie creatures. There are tons of them.