It's night, and you've just peed in the toilet, but there's a problem, the toilet cistern is not full, and it's too late to wait, what are you going to do and for what reasons?
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Wait until tomorrow for the cistern to fill so you can get rid of the urine.
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Pour water from the cistern, even knowing that it is not enough to get rid of urine.
Ok, but siphonic systems still flush just fine if you pour water into them..? I’ve been doing it my whole life. Like even if you just pour water from a pitcher at a normal rate it’ll eventually hit a pressure point and flush itself (assuming there’s no clog). If you do it from more than a foot above the bowl it flushes basically instantly.
An instant flush is what OP wants. If they can do that with the siphonic system, that's great.