The impeller does pretty much the same thing as the spray nozzles in a wash-arm, it delivers water at high velocity toward the dishes to clean them. The whole tank is NOT filled with water... like a regular dishwasher there are 3 - 6 gallons in the bottom. The impeller spins at high speed, like any pump or fan, which flings the water up when it comes in contact with the impeller.
There are a number of videos on Youtube which show this type of machine (like this one)
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I hops this helps!
Kevin