When a man and a pregnant woman are enjoying a romantic evening sixty-nining while the man is on his back and the woman is 8.5 months pregnant. Since her stomach is too large and in the way both man and woman can't engage in oral cherishment simultaneously.
The woman will have to rock back and forth on her baby filled stomach so man and woman can take turns fellating and cunnilinging.
May also be referred to as "Lumberjacking"
Man 2: "Me too - let's see-saw her!"
Child 2: No, I like the slippery dip better.