Ok, this is a very weird thing to ask...but a friend just did so to me, and I really had no idea beyond a guess of pressure.
The general toilet, when functioning properly, just to be safe, here in Tucson, AZ...you add water to it, and it automatically lowers itself/drains to a certain point. Is this done by pressure? I really see no other way, but not all too sure how it uses that to determine the volume...down there. A flap or something?
I know, werid question, but he keeps bugging me
(PS: I never asked this. Post self-terminating 10 seconds after you answer it and I read your answer. Muahaha).