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I'm a hedgehog, not a fox, so I will tell you the one big thing I learned at the Forge: that when you say a thing is true in the game-world, players will have their characters act like it is true.
I had written a game for one of the early Game Chef contests, in which the characters belonged to hunter-gatherer tribe in an icy world of eternal night ruled by spirit-like stars, but the dawn was approaching and the PCs, as members of the tribe, were going to have to deal with enormous changes in their world.
To give that scenario teeth, there were these tribal resources that determined the shape the village was in--oil, bone, meat, and hide, I think, each with different effects if it were understocked or oversupplied.
The thing is, it was not obvious to players what the numbers meant in terms of motivating them to act. Oil 3, Bone 2, Meat 2, Hide 6--what does that mean to me, Angyrok the bold shaman? Without more explanation, nothing. Why not start with the explanation?
So I ripped out that system and started using cards with oracle-like motifs on them. "What's going on in the world, GM?" GM throws a card: "Hunting Camp: To set things in order, as in defense." "Angyrok, the men of the village are despondent and fearful! The animals out on the ice have grown menacing and aggressive, and several men have been injured while hunting. What do you do?" That player, if he's the bold shaman I think he is, is either going to whip those men up and get some hunting done, or go find the angry bear spirit that's riling up the animals and fight him to the death. Regardless of what the character does, or even if the character does nothing, something interesting will occur--and all because I told the player it was true.
You might call dignify that by calling it a corollary of the lumpley-Boss principle, or even just say it's merely an application of it, but to me it felt like a real insight. And it came out of engaging with game design from a Forge perspective: you can make the game that does what you want.