litotes; meiosis
A rhetorical device where an affirmative is expressed by negating its opposite, often for understatement or ironic effect. Technical term used in literary analysis.
The expression 'not bad' is an example of litotes.
His speech made frequent use of litotes.
修辞法 is a general term for rhetorical devices; 緩叙法 is a specific type.
From 緩 (gentle, moderate) + 叙 (describe, relate) + 法 (method, rule), a calque of the Western rhetorical term 'litotes'.