A character or characters that enjoy having a lover insert their fist inside either their ass or pussy, and fuck them with their fist until they orgasm intensely. Fisting is always worked up to, beginning with finger fucking heavily with lube, adding fingers until the whole fist is thrust inside. There is a lot of description of the filling sensation, building an intense and powerful orgasm that ends with an explosive messy orgasm by the receiver, and the giver is incredibly turned on by doing the fisting as well.