When two girls scissor and one girls vaginal area migrates back towards the anal area of the other scissor-ess, and that girl then flatulates into the vagina of the former scissor-er. Then the two finish and then the girl containing the methane in the uterus then releases with such great force that it shakes the bed.
Other variations include a lighter being used to ignite the said methane. If there is an accident that occurs, such as pube flambe, please consult your doctor.
So the other night, my girlfriend farted in my pussy... That was a weird quief; it sounded like The Everett.