How a Toilet Siphon WorksBy
To understanding how a toilet siphon works, think about a train going over a mountain.
As American Standard Chinaware Director James Walsh explains in this video, once a train crests the mountaintop, it heads downhill, pulling the rest of the train with it.
In a toilet, the flushing action pushes water and waste up the s-shaped trapway, until it crests at the top, the part known as the weir. The water then rushes downhill, creating a pull that scours waste and paper from the side of the bowl and out the trapway.