I think one way of thinking about what woody is saying is to realize that with an electronic throttle and complex engine control software you can meter the air so that even if it is being compressed (which a twin screw always does by design) it can still be less than the engine is using at part throttle cruising. This is the definition of a vacuum, a pressure differential. so yes the charger is spinning at an rpm that is a certain ratio to the crank pulley, but if you supply it with very little air, it wont actually send a boosted charge to the pistons. also the intercooler pump will always run because if the coolant was just sitting there it would heat soak even if you never hit any boost, just from the heat of combustion that is conducting through the engine block and all metal parts bolted to it.