A flatcocker (see 'flatcockers') who ALWAYS assumes (or wants to, anyway) the dominant position. And, unlike a flatcocker, who usually only uses the prehensile pubic and hip bones, the bullcocker sometimes enlists the help of instruments to further her mission and to better give satiation to the submissive femme that is the object of her affections.
"Did you see how that bullcocker worked over that submissive femme in the movie? I didn't know if that little lady would be able to handle that onslaught. That REALLY turned me on!" 'Bullcocker', as well as the word 'flatcockers' has been in use for many, many years here in the Southern United States.