Ki (or Spirit)
For game purposes, this is grossly oversimplified as spirit energy and quantified by Ki points1). Characters have a pool of these, starting with one at character generation, that they can spend to gain an advantage on a dice roll (i.e., roll twice and take the best result). Characters who entirely deplete their Ki reserves will recover one following a good night's sleep. Otherwise, Ki is awarded by the GM fiat for positive supernatural interactions and play rewards. For example, all my players received a point of Ki for freeing a lake spirit that had been imprisoned by someone believing themselves the servant of a demon god.
In narrative terms, this represents a character's ability, through martial, meditative, or magical training to focus their spirit into mundane tasks to achieve exceptional results when the pressure is on. They are required to state they are spending Ki before they roll.
This article first appeared in Casket of Fays Issue 8.