For a turbine to make boost pressure it is necessary a flow of exhaust gases to pass through it. It is the only way that the exhaust gases on a turbine-equipped engine can go out. So, exhaust gases goes out and the turbine rotates. If it rotates then it generates pressure in the intake manifold.
What the ECU (Engine Control Unit) has to do with this? Apparently nothing. ABSOLUTELY WRONG. Example: BMW 530d 2003 common-rail diesel engine with a lot of engine power loss. With diagnosis software I sow this error code: boost pressure control ? deviation. On actual value data it shows: at 850 rpm boost pressure was 1000 mbar, at 3500 rpm it was 2300 mbar. And the vehicle was standing still. On drive tests boost pressure raised very quickly and ECU forced to keep it at 2200 mbar by cutting out fuel injection (loss of power).
So, we come back to the basic: on low and partial engine rpm (not to be confused with engine load) the volume of exhaust gases is relatively low and the turbine generates a normal pressure between 1000 mbar-2200 mbar. On high rpm the volume of gases is very high and the turbine would rotate so quickly that it could produce so much pressure that it could explode the hoses and inter-cooler radiator.
The solution: deviate a part of exhaust gases from the turbine and lead them outside. Deviation is performed with the help from a vacuum controlled actuator. The vacuum applied to the actuator is controlled by the ECU through a vacuum control solenoid. To keep boost pressure to a normal operating condition ECU receives information from air mass meter, injectors, engine temperature, oil temperature sensor, air temperature sensor, gear selected (automatic transmission only), climatronic, pedal position sensor, etc.

The vacuum actuator on BMW was mechanical blocked in max. supercharge position.