The cranial and spinal cavities are subdivisions of the cavity.

when you rub, you also apply pressure, that pressure can urge the poop to come out. i think that when your rubbing, you also might be slightly breaking it up and that can further fill the stomach.  

one time i was looking at my neighbors screen door to see if the door was open i was squinting like a fool for 10 minutes before realizing he was starring back i then died from embarrassment and am now typing this from the ghost world