It quotes what someone said and then sets the reality beside it, almost always a contradiction: you said you were dieting, so where did the red bean porridge come from; you said you were going to look round the room, so where did you go; the YouTuber who said he would sleep and has not stopped talking. It can also mark a saying coming true, as in the line about speaking of the tiger. The gap between the claim and the outcome is the whole point.