Litotes, a particular form of understatement, is generated by denying the opposite or contrary of the word which otherwise would be used. Depending on the tone and context of the usage, litotes either retains the effect of understatement, or becomes an intensifying expression. Compare the difference between these statements:

• Heat waves are common in the summer.
• Heat waves are not rare in the summer.

Johnson uses litotes to make a modest assertion, saying "not improperly" rather than "correctly" or "best":

• This kind of writing may be termed not improperly the comedy of romance. . . .

Occasionally a litotic construction conveys an ironic sentiment by its understatement:

• We saw him throw the buckets of paint at his canvas in disgust, and the result did not perfectly represent his subject, Mrs. Jittery.

Usually, though, litotes intensifies the sentiment intended by the writer, and creates the effect of strong feelings moderately conveyed.

• Hitting that telephone pole certainly didn't do your car any good.
• If you can tell the fair one's mind, it will be no small proof of your art, for I dare say it is more than she herself can do. --Alexander Pope
• A figure lean or corpulent, tall or short, though deviating from beauty, may still have a certain union of the various parts, which may contribute to make them on the whole not unpleasing. --Sir Joshua Reynolds
• He who examines his own self will not long remain ignorant of his failings.
• Overall the flavors of the mushrooms, herbs, and spices combine to make the dish not at all disagreeable to the palate.

But note that, as George Orwell points out in "Politics and the English Language," the "not un-" construction (e.g., "not unwilling") should not be used indiscriminately. Rather, find an opposite quality which as a word is something other than the quality itself with an "un" attached. For instance, instead of, "We were not unvictorious," you could write, "We were not defeated," or "We did not fail to win," or something similar.