It's hard to explain in the abstract, but it's something that I am certain you will recognise if you have read the kind of modern, indie games in which it is most used - particularly, for some reason, introductory sections on what role playing represents.
Take Mythender, for example, which is to my mind probably the paradigm case. This is more or less from the first substantive page:
Or you’ll tell the stories of how you ignore all that humanity business and go pedal to the fucking metal, diving headfirst into the heart of Myth and leaving piles of corpses in your wake. You’ll gain power by making mortals to worship you as a god. Eventually, your comrades will be forced to murder your Mythic ass.
Spoiler alert: they totally fucking will. [...]
Mythender is about kicking ass and erasing names to a heavy metal sound track, about dancing on the knife’s edge between having the power to slaughter abominations and becoming an abomination yourself. Go do that already
See what I mean? "Go do that already." Deliberate RPG Rulebook Sass, turned to 11.
Sometimes it manifests itself in a slightly smarmier fashion. See, for instance, this section from Houses of the Blooded:
If you’ve ever played a roleplaying game before, you may have noticed that characters seldom age—locked in a perpetual state of twenty-five years old—and they always seem to “get better.” As they move through their lives, experience points always add to the character’s abilities. Regardless of how old they look, all RPG characters seem to be an eternal and everlasting twenty-five years old.
Not so here.
Leaving aside the sniping at D&D, it's that final coda - "Not so here" - which turns this into Deliberate RPG Rulebook Sass. Picture the author wagging his finger at you with a slight smirk playing across his lips. "Not so here." There's that slightly patronising subtext which is the mark of true sass.
The patronising subtext can become the supertext when the Deliberate RPG Rulebook Sass becomes overt. Here's a section from the introduction to Apocalypse World:
You probably know this already: roleplaying is a conversation. You and the other players go back and forth, talking about these fictional characters in their fictional circumstances doing whatever it is that they do. Like any conversation, you take turns, but it’s not like taking turns, right? Sometimes you talk over each other, interrupt, build on each others’ ideas, monopolize. All fine.
All these rules do is mediate the conversation. They kick in when someone says some particular things, and they impose constraints on what everyone should say after. Makes sense, right?
Makes you feel like a 9 year-old, right?
Now, I don't want to be misunderstood - I like Mythender and Apocalypse World, and Houses of the Blooded is interesting if not exactly my cup of tea. Please think of this post less as a critique or a snipe, and more of a plea to the future game-designers out there. Keep the sass to a minimum and just write nice, simple, plain, non-sassy and non-patronising, yet evocative, English like this:
Before television, there was radio. Audiences earlier in this century sat in front of their radios
and thrilled to the exploits of bigger-than-life radio heroes. Since it was radio, they couldn't see
what was going on, but they didn't need to—all the action was described by dialogue, narration, and sound effects, and was translated by the imaginations of the listeners into scenes they could see, experience, and remember.
Role-playing games are much like radio adventures, except for one important detail: they're interactive. One player provides the narrative and some of the dialogue, but the other players, instead of just sitting and envisioning what's going on, actually participate. Each player controls the actions of a character in the story, decides on his actions, supplies his character's dialogue, and makes decisions based on the character's personality and his current game options.
Go do that already.