When you are flipping a bitch, you are making a U-turn. I have no idea how or when this phrase originated but it seems oddly appropriate for the maneuver it describes. U-turns are definitely bitches; especially when there is heavy traffic, a lack of power steering or a multitude of idling pedestrians involved. Most everyone who I've heard use this phrase with frequency has been from or culturally influenced by the Los Angeles area. I guess that it's, for now, a pretty regional term.