My son had it on his 350bhp Focus RS.
The Water/Methanol mix, sprayed into the inlet side of the intercooler increases the cooling effect of the intercooler, making the inlet air denser.
Then a second injection of water methanol is injected into each throttle body, so when it gets into the cylinder on the ignition stroke increases the effective octane rating of the petrol allowing a higher boost pressure.
As a further bonus the water is converted, explosively, to steam in the cylinder, increasing power.
In addition to adding power it made the burn much cleaner, so that without a functioning catalyst the emissions were well inside the required limits.
