I think that there should be some sort of rules mechanical 'contest' that ends with the opponent getting buried. And possibly how well the contest went would determine how well it worked. In general, I think that things that are important to the story should also have some sort of mechanical 'support' to back them up.
As some sort of samples of this:
You are fighting the bad guy at the edge of the cliff. 'Getting thrown off the cliff' is how the fight ends, but to actually get to the end, you have to run a regular combat first. You can 'just' pick up the bad guy and throw him off the cliff.
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But do things this way? So that both the players and GM don't have to have above game arguments about how effective their attempt at knot cutting was, they can run a mechanical contest to decide.
So: example of how you could resolve your building collapse question:
The situation:
The PCs have prepared the building to explode, and have lured the baddie here to bury him under it. How to decide if their idea is effective or not?
I'll say that I'm going to run this as a maneuver contest, where each of the PCs gets to come up with something they did for the prep, which will set a difficulty for the bad guy to overcome with his compensating roll.
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GM: so what did you do during the preparation to make sure this worked:
PC1 (the mad bomber):
I, of course, prepared all the explosives! They are well placed, of sufficient power, and hard to notice.
(declares a craftsmanship (4) maneuver plus has a mad bomber stunt that adds +1)
Rolls: (-, -, _, +): total: 4
GM: the baddie rolls his investigation (3) to try to notice your concealed explosives:
Rolls: (+, +, -, _) : total: 4.
GM: He notices the explosives,, but only at the last moment, and he rushes for a stairway to get out of there...
PC1 (The naughty hedonist)
As the baddie runs into the stairway, he discovers that all the stairs have been liberally coated with... lubricant. looks smug I need to know where to get such things in quantity.
(makes a resources maneuver(4), and invokes his Naughty Hedonist aspect)
Rolls: (-, +, -, +) total: 6
GM: Well, that was... unexpected. The baddie will try to run up the stairs, despite their unexpected slipperyness.
(makes and athletics maneuver(4) and gets a +2 for having inhuman speed, and this being a sprint action)
Rolls: (+, +, +, _) total: 9
GM: Alas, despite the difficulty, his infernal speed lets the baddie race up the stairs in hardly any time at all.
PC3: (The gun nut)
I'm waiting across the street, to blast the baddie back into the building. Sure all this firepower won't hurt him, but I have Hollywood physics on my side! All those bullets should send him flying backwards.
Rolls his guns (4) skill, and invokes his high concept and all the firepower of a Texas militia aspect.
Rolls: (+, -, -, _) total: 7
GM: The baddie attempts to stand unmoved, despite your fusillade.
an might (4) maneuver
rolls: (_, _, -, _). total: 3
GM: but he isn't successful, your storm of fire knocks him back into the exploding building.
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GM: now, how long is he going to be in there? You have one tie, and each beat the other once, so he won't be trapped under there for more than a few days. Better come up with something else...
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So, I gave the PCs the feeling of effecting the outcome, which probably makes them happy.
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[edit], for more commentary:
Note that you don't have to invoke aspects before the other guys rolls when you play at the tabletop, you can totally use them afterwards. But it's a common convention to more or less require aspect invocation ahead of time in pbp games to reduce the time required to resolve actions.