Spiro, the standard toilet that most of us probably know, have the valve
in the bottom of the tank operated by the flush handle.
The valve less have a kind of tank or bucket inside the tank that tilts to
dump. The push to make water saver toilets resulted in small tanks that didn't
work and inserts into older ones that didn't work right. They displaced water
volume in order to displace water usage.
The newer ones kind-a have the same tank size and some have the same water
volume; but, only the top part of the water is used. The water, located in the
upper portion of the tank has the power and momentum. The water mass gives
pressure even if it is not all used and the height of drop adds power to flush
faster.
I hope that makes it a little clearer and not muddied the whole idea. The
real old toilets with a water tank near the ceiling always flushed good with a
smaller water volume plus a roar and a half. Do you remember those? It would
not surprise me if they came back in designer fashion.
....................bob
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