When two partners engage in
sexual intercourse and the male performs
cunnilingus on the female during her
menstrual cycle, he inadvertently collects some of her uterine lining into his mouth. As he finishes the oral sex, the two partners kiss, and the male spits the detritus of blood and menstrual chunks back into her mouth. This is the art of "meatballing," as the chunks represent what looks like a meatball.