Why does a toilet flush rush away and then stop?

A toilet bowl keeps a small pool of water even between flushes. Its outlet curves upward and then down toward the drain. Water remaining in this curved trapway makes a seal that helps keep sewer gases out of the room.

In the siphonic tank toilet shown here, opening the flush valve sends stored water rapidly into the bowl. The surge fills the trapway and pushes out much of its air. A continuous water path now runs over the bend and down to the lower outlet. Gravity acting on this draining water establishes a pressure difference that draws more bowl water into the path: a siphon.

As the bowl empties, air enters the trapway. The continuous water path breaks and siphoning stops. The tank valve closes, and the fill system replenishes the tank and bowl. A float stops the refill at the intended level. Not all toilets flush by this mechanism: washdown, pressure-assisted and other designs arrange the flow differently.