because you have a waste gate and a surge valve and a blow off valve............ waste gate opens when at idle surge controls the boost spikes associated with the undersized turbo for the car and of course the blowoff vents the air in the line fast when the car goes into vaccum so as to stop the impellar shaft from being snapped from compressor surge
i should get my friend to answer he knows alot more but i think my regurgicated info is close to being correct
The wastegate is closed at idle as it is spring loaded it only open when pressure from a intake source reaches a set pressure, this allows air to bypass the turbine and control boost.. The reason there is not always boost is simply engine load. When you are cruising, the incoming air is being processed and expelled at a low enough rate that the car can consume the incoming air without intake pressure building. Boost is nothing more than a intake restriction.
The blowoff valve is a pressure relief valve that is spring and vac/pressure operated, once you life the throtthe it sees the pressure differential and momentarily vents the excess intake pressure to prevent compressor surge.