The term 'understatement for rhetorical effect' can be summarized by the word LITOTES, which is a figure of speech consisting of an understatement in which an affirmative is expressed by negating its opposite. LITOTES is used to emphasize a point by stating it in a more subtle way, often to create a humorous or ironic effect. It is derived from the Greek word for 'SIMPLE, ' and is used to make a statement more powerful by understating it.