Motion input
A special move triggered by a sequence of directions — the quarter-circle for a fireball, the Z-motion for an uppercut — rather than a single button.
Street Fighter (1987) introduced the idea; Street Fighter II made it the genre standard. The classics are the quarter-circle down, down-forward, forward (often written 236 or QCF) for a fireball, the Z-motion for an uppercut, and the full circle for a grab. The engine keeps a short ring buffer of recent directions and fires when the pattern completes within a handful of frames.
The whole craft lives in the margins: how many frames each step may take, whether extra inputs are tolerated, whether shortcuts are accepted. Too strict feels unfair, too loose fires by accident. Newer games layer on "input leniency" and shortcut tables to meet players who never learned the arcade stick's precision.