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.
An instant flush is what OP wants. If they can do that with the siphonic system, that’s great.