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Water (H2O) is effectively an inert molecule during combustion. It does cool the charge (very high heat capacity) during compression and can help efficiency and detonation resistance. But in the actual combustion event it pretty much just takes up space. This is effectively a way to simulate an engine of smaller displacement at part throttle cruising. At a constant speed it would be more fuel efficient if you could magically trade for a smaller engine running at wide open throttle (WOT). Injecting water vapor at this time approximates this. The water molecules take up space and require opening the throttle wider to account for this but the system knows not to add additional full during these conditions. Actually less fuel is required since the wider throttle opening has less pumping losses. When full power is needed simply turn off the water vapor and you "magically" have your full displacement engine back.
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