"Flapping" in the context of a porn video tag refers to a specific sexual act or movement performed by one or both partners during a sexual encounter. It usually involves quick, rapid, and forceful motions of the penis or tongue in and out of a vagina or anus, often creating a flapping sound due to the wetness or lubrication involved. This tag is typically associated with analingus or penetrative sex where the penetrating partner's movements are fast and intense, creating the "flapping" sensation or sound.