This term is still in it's infacy, but I would like to spread the usage of it.
The term "oyashiro-mode" describes the description of the character in my avatar - Ryuuguu Rena from "Higurashi no Na
A normally cute and lovely character, who suddenly becomes psychotic in an instant triggered by a certain shocking event. This trait is often associated with cold smile, blank staring eyes, psychotic actions (like stirring in an empty pot), sudden bursts of euphoria and anger, etc.
Note the similarities between:
Ryuuguu Rena from "Higurashi no Na
ku Koroni"......................................and that famous shot of Fuyou Kaede from "SHUFFLE!"
[I-know-it's-bad] Usage example:
I must've pissed in my pants the first time I saw Rena's oyashiro-mode