Litotes

Litotes is "a figure of speech and form of irony to emphasize a point by stating a negative to affirm a positive," and "a form of understatement." The mechanism is the double negative: not bad for good, not unlike for like, Queen Victoria's "We are not amused," the Beowulf poet's sword that "was not useless." The denial understates and the reader compensates, which is why such negatives "often have the paradoxical effect of exaggerating the statement." The figure can also work "euphemistically to diminish the harshness of an observation."

For the novelist litotes is voice: a narrator who will not praise directly, who would not say they were uninterested, characterizes faster than a paragraph of confession. It is the mirror of hyperbole, overstatement turned inside out, and it shades from irony into euphemism whenever the negation softens a hard fact rather than sharpening one.