1. exertion or effort directed at taking turn penetrating one's partner as part of a sexual activity.
2. something on which exertion or labor is expended to achieve mutual penetration, in turn.
3. to move in agitation, usually under strong emotion, in order to penetrate one's partner, and switch position under emotional agitation in order to be penetrated.
4. to take turn fucking each other (implying both partners have the apparatus to achieve the objective intended).
I had a blast in college where my roommate and I would flip fuck doggy style or missionary whenever we could.