Understatement
Deliberately presenting something as smaller or less serious than it really is.
Understatement plays down a subject to create irony, humor, or a sense of composure under pressure. Writers use it so the gap between the calm words and the real stakes registers on the reader. It is the opposite impulse to hyperbole.
Example
Holden’s flat, offhand way of describing painful events downplays his distress, letting readers feel how much he is holding back.
The Catcher in the Rye · J. D. Salinger
See it in action
Analyses on StoryBites that use understatement:
The LotteryShirley JacksonMiseryAnton ChekhovThe KillersErnest HemingwaySredni VashtarSakiA Respectable WomanKate ChopinRegretKate ChopinThe Boarded WindowAmbrose BierceIn Another CountryErnest HemingwayThis Way for the Gas, Ladies and GentlemenTadeusz BorowskiThe Sky Is GrayErnest J. GainesIndian CampErnest HemingwayA Farewell to ArmsErnest Hemingway