Henshin

Henshin is 変身, change of body. As a plain noun it covers any transformation, including a caterpillar's. As a genre term it names the moment a character becomes their hero form, and, in costumed hero television, the word actually said out loud to trigger it. The pose comes with it. Arms out, one turn, hold the last frame.

Kamen Rider fixed the convention in 1971. Shotaro Ishinomori's hero shouted the word, the belt caught the wind, and Japanese children spent the next fifty years doing the same gesture in living rooms. The habit crossed into animation quickly, and magical girl series turned the sequence into a set piece with its own music cue, its own colour scheme and its own stock footage.

The stock footage is the practical half of the story. A transformation runs twenty to forty seconds, is animated once at high quality, and is then reused every episode for a year. That single decision buys an entire season's worth of budget for the fights. Audiences noticed decades ago and watch the sequence anyway, which tells you the cost saving turned into a genuine pleasure.