Matthew Conway recently wrote Fluff and Crunch Are Dead To Me, about how he’s grown to hate the terms, but I see them as getting at something. To me, anyway, Crunch is all the mechanics of the game: you roll this, and subtract that number from this other thing, if the result is 0 or less, the creature is dead, and so forth. Fluff is all the stuff that doesn’t touch the mechanics at all, and could be freely swapped with any other fluff without changing the in-game result. To take a concrete example, if you know the HERO game system: that an attack is 6d6 Energy Blast, Armor-Piercing, 1/2 End Cost, Activate 14- is all Crunch. It tells you everything mechanical you need to know to resolve the attack, and absolutely nothing at all about what the attack is or how it appears to the characters. The fact that it’s a bolt of flame, or darting daggers of ice, or even a pack of pink bunnies that materialize, savage the target, and disappear is pure Fluff, flavor without any substance.
Now, neatly separating things into Crunch and Fluff is a huge convenience to the game designers, who can on the one hand say “Hey, I don’t need to write any special rules for Ice Daggers versus Fireballs, an Energy Blast is an Energy Blast is an Energy Blast…take some advantages or limitations if you want it to have a different mechanical effect” and on the other can say “Here’s an adventure you can use for any system whatsoever, just plug in your favorite mechanics and go.” It’s also a convenience for the player and GM insofar as it makes the rules streamlined and elegant and lets them use this or that material with their favorite system.
But… it’s not a pure win… at least for players who are interested in having the rules closely track the game description and story. See, unless you’re approaching it as a board-game, almost everything that actually interests the players is at the level of description. What they want to do is toss their Fireball at the bad-guy and see the fur fly (or singe); rolling the 6d6 and subtracting the target’s Energy Defense divided by 2 while ticking off 3 endurance spent is just a means to the end, and the end is telling them what happens next when they throw that fireball. But when the game designer has severed the link between mechanics and description, which is what designating them as crunch and fluff is mostly about, that can make the interface…mushy and undefined. In extreme cases (cough 4e cough) the player can lose the sense that they know what’s actually happening in the game world to cause the mechanical effect, or worse know that the description is just “flavor text” and ought to be ignored lest it give you the wrong impression of what ought to be possible in the game world. A clean separation of crunch and fluff makes it impossible to reason from the level of description.
So what players often would prefer…you’re way ahead of me here, I’m sure…is a less clean separation, what I call “fluffy crunch” and “crunchy fluff.” Fluffy Crunch would consist of making every bit of crunch have a visible, comprehensible description-level corresponding bit of fluff. You don’t just Soak a wound, you desperately twist out of the way so that it just grazes you.
Crunchy Fluff is making sure all the description-level stuff gets reflected appropriately in the mechanics: If your super-power lets you created Ice Daggers out of nothing, you darned well should be able to create one and use it to cool your drink, or ice-down a twisted ankle. No saying the rules don’t support that that just because the crunch description doesn’t allocate a +1/256th advantage “Can be used to cool physical objects in a non-violent fashion.” Your ice daggers might get a bonus (or a minus) versus fiery creatures, or be easier to generate in artic conditions and harder in the middle of the Sahara, but in any case shouldn’t be indistinguishable from your companion’s Laser Pistol.
Crunchy Fluff also comes about from making the mechanics support the details of the setting. If vampires in your setting are unable to enter a dwelling without an invitation, it helps to support that with actual mechanics: is it an absolute prohibition? Can a sufficiently powerful vampire overcome it? If so, how? A Will roll? Or is it something that the vampire can do, but it will have consequences. Will it take damage for every turn it remains uninvited? Can an invitation be revoked? If it can, can the occupant just say the words, or does the occupant have to engage in some kind of test of wills? This kind of tuning the rules to reinforce the description of the setting is an important way of making it feel like the setting has “heft”…that the adventure that the players are on couldn’t just be “re-skinned” (to use a computer gaming phrase) with the vampires being replaced with killer androids or cattle rustlers and nothing else but the fluff changing.
If you try to write something as pure Fluff, that can be applied to any setting, those are the kinds of things that can come back to bite you, no pun intended. If the adventure assumes that vampires can’t enter a dwelling without an invitation period, but the system mechanics say that any sufficiently powerful vampire can…and the adventure has a vampire that’s supposed to be one of the most powerful in the world….
In any case the rules should be used to support the description that’s the heart of play. Fluffy Crunch is there to give the mechanics a reason and a description; a neat mechanic is not self-justifying, even if it does give the player something extra to think about in terms of winning the board game. Crunchy Fluff makes the descriptive level of play have consequences as well as consistency. Both are important to a satisfying RPG, and IMO both are preferable to designs where one is divorced from the other.