The sanity trait is generally only included in anthologies which are set in a horror genre, where a character might encounter things especially frightening or disturbing. Sanity rolls are made to see how the character is able to deal with the terror and strangeness of such situations, and failure can mean bouts of uncontrollable fear, the development of phobias, or even complete madness.
Sanity rolls are similar to health rolls, only instead of your character's physical health being threatened it is her stable mental state. Sanity rolls may be called for by the Storyteller in any situation where your character sees or experiences something truly horrible.
As with other traits in the Window, sanity is a only a tool which helps the Storyteller present the story in an effective way, but like wounding, it is up to the actors to make the situation come to life. (Don't forget about the Second Precept.) A good horror storyteller will have the actors on the edge of their seat with tension and fear already... sanity is a mechanic which reflects the dark consequences of facing the supernatural in a somewhat more concrete way.
When the Storyteller calls for a sanity roll he will set a target number which you must roll equal to or under with your sanity die.
The target number itself is of course based on how powerful the horror is which is affronting your character. For instance, seeing a carcass on the road might call for a roll with a target of 6, while being surrounded by cold, rending tentacles which are sliming through your hair and trying to crawl into your mouth might call for a 2. (Trust me.)
Again, it's up to the Storyteller and the exact situation.
A successful Sanity Roll means you are able to think through the horror and keep control of yourself. Failing a sanity roll can mean several things. One of the most common effects is to freeze up and stare. Another is to flee in terror, or uncontrollable screaming. Another is to fall to the floor and roll into the fetal position. Yet another is to be cursed with a permanent phobia. All these wonderful things and more await your character should they not make one little roll. Sometimes the Storyteller will "take control" of your character for a brief period of time during their fugue, though often they will leave the exact effects of the failed roll up to you.
Oh yes, and don't forget about your Sanity trait dropping a rung
on the competency ladder. That can happen, too, just as it happens
with health. If your sanity should spiral down until it drops
completely off the competency ladder, then your character has
gone insane, which is effectively just as bad as death...
Once a character is insane, they become "property" of the Storyteller for him to do with as he pleases. Perhaps your character will run off into the corn field, only to come back in some future chapter, or simply kill himself, or maybe he'll be taken by the other characters to an insane asylum.
It is possible, with the proper care and possibly hospitalization, that your character could be brought back to the land of the sane, to be used again as your character at a later date, but that's up to you and the Storyteller to discuss.
Remember the Third Precept, and consider how interesting it can be to have one of your former
characters locked up in an institution somewhere, to be visited
and given fruitbaskets on the holidays. Sanity can provide very
enjoyable plot twists, and when used sparingly it can help add
a rich layer of fear to your horror anthologies.
Sanity Roll Example:
"Run, Damn it!" Deron Jones stood shouting at his companions, the writhing Darkness before him. His hand held his government issue .45 tightly, white knuckles on black metal, hot from the spent clip.
A tentacle thrust from the mass and coiled tightly about his ankle. His mirrorshades fell from his face as he was pulled to the wet floor with the shatter of glass and plastic. The Storyteller shook his head. "Sanity check, target of 3." Deron's sanity had been damaged before, and he was already bordering on a nervous breakdown (D30).
A die was tossed: a 7.
Deron's eyes gleamed bright with tears. His howling was the last thing his companions heard as they fled screaming through the door and slammed it behind them. The twisting moistness was around his hand now, his neck, his face. In the pit of his mind he knew that he should struggle, to break loose from the embrace. But he could do nothing but scream.
Give me another sanity roll," said the Storyteller. "Target of 5. This one's for real." Deron felt like he was dead already. It was under his clothes now, pulsing and cold. His muscles were reacting violently, spasming. A second die was tossed and came to a rest: a 14. Something inside him... snapped.
"Deron's gone now," said the Storyteller. The others were to the van, cursing and swearing. The realization of their companion's fate chilled them to the bone.