To insert your whole fist in a(nother) womans's vagina/a(nother) man's/woman's anus. (This is known as handballing).
Vaginal fisting is - in the author's opinion - INCREDIBLE if done correctly, but handballing is not - and never will be - in their repertoire.
Whilst inside Sasha, Kristen swivelled her fist round and round, causing Sasha to moan in pleasure. Then, Kristen changed her hand into an actual fist shape, alternating between opening and closing it inside Sasha, who was now screaming for more, and swivelling the fist shape inside her.