Humor disarm works because a joke does two things at once: it makes the source more likeable, and it occupies your evaluative attention with the pleasure of the humor, leaving fewer resources for critical counter-arguing. The observable move is to wrap a serious ask, a dubious claim, or a pointed message inside comedy — a funny ad, a witty brand voice, a meme, a “just a joke” aside — so you are amused, and warmed toward the source, at the moment the real payload lands.
A distinctive sub-form is the “just joking” wrapper, which adds deniability: if the claim or insult is challenged, the speaker retreats to “it was only a joke,” making the message hard to hold to account.
It differs from simple liking / rapport building (T8.4), which need not carry a payload, and from pure entertainment, which has no embedded ask. The manipulation is not the humor itself but the use of amusement to move a claim past evaluation.