By peristaltic muscle contractions (a wave of muscle contractions down a tube). Really, your body uses both muscle contractions AND gravity. So you're making it harder. But the muscles can still do it alone.
“Everybody has a secret world inside of them. All of the people of the world, I mean everybody. No matter how dull and boring they are on the outside, inside
them they've all got unimaginable, magnificent, wonderful, stupid, amazing worlds. Not just one world. Hundreds of them. Thousands maybe.” -Neil Gaiman
Me and a friend once wondered how much longer it would take for a person to go to the bathroom when said person drinks all his beer while upside down. Thinking it was a good idea, we put it to practice, thinking we could drink much more before needing to go take a leak.
The result however was far from what we had expected.