A clenching motion of the anus. A character's anus may be shown clenching or contracting involuntarily as they orgasm (sometimes repeatedly).
The anus may be shown clenching prior to sexual activity, either reflexively, or by the character intentionally causing their anus to "wink" while presenting hindquarters and spreading their cheeks. In other cases the anus may be shown dilating wide and contracting after being anally penetrated by a partner or object.
Outside of animations, this is typically depicted by showing emanata around the anus, usually paired with "wink" "twitch" or "clench" as onomatopoeia, or showing the anus in a normal state and clenched state in sequence.