I think it is all in this thread Basically, turbo engines run rich underload as a safety measure. This keep the engine cooler as the excess fuel helps with cooling and I think with detonation too. The problem with this extra fuel is that it causes inefficient combustion, so you loose power. By removing the extra fuel under load, you get more power. The water injection then takes the place of the fuel and cools the engine and helps with detonation.
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