there are a few ways, first is a cold start valve injector setup, about 125 quid, the other is more expensive around 300 for a mf2 one, Both needs setting up as with most mods especially anything to do with fuel.

Not sure about a power gain, its also there to provide enough fuel to match with your airflow at a point where the orginal system cant cope, usually high boost cars,

They cut in at a desired level 15psi etc, they provide a good on and off fuel ecomony