Place it immediately after the unbelievable statement, then follow with the Ceeday Huh for a second reaction, or lead into it with Sudden Suspense when a reveal is about to turn out false.
About this sound
This one-second clip is a single spoken Japanese “Haa?” delivered in a stretched, questioning voice, as if the speaker has just heard something absurd. It feels sharply sceptical and funny, turning a claim into an instant challenge. The sound comes from Japanese reaction culture, where that stretched vowel signals disbelief without needing any more words.