Re: How does the toilet work in terms of water flow?

A good site that explains toilets is at  howstuffworks.  The starting 
of the siphon is 
best explained with water momentum and not hydrostatic pressures, just 
like truing to explain water hammer in pipelines is not a simple hydraulic 
analysis.  In order for the siphon pipe in the toilet to work, all of the 
air must be forced down the drain.  This is why when a cup of water is 
dropped in the bowl, nothing will happen because it will raise the level 
in the siphon pipe only slightly and spill over into the drain, air will 
remain at the crest of the pipe and the siphon will not start.  When a 
large volume of water is dumped in the bowl, there is the momentary raise 
in the level of the bowl that will start the momentum or pressure wave to 
expel all of the air in the siphon; and as discussed at the howstuffworks, 
there is also an extra water line directly into the siphon pipe that aids 
in expelling all of the air.  Once the siphon starts, it is a simple 
hydraulic analysis that shows that the water in the bowl will be sucked 
out until air gets back in the siphon pipe and breaks the siphon.