The turbo is essentially a fan, it will spin FASTER if the blow off valve is closed. If the valve is open it will have to move more air making it more difficult to turn (spool slower). It is kind of difficult to explain. Take a hair dryer and turn it on, cover the discharge up completely and the motor spins faster (there is no load on it as it is moving no air) and it is actually drawing less current. To sum it up we have used BOV on BIG V8 turbo cars but only to control the boost at the starting line to keep it from running away while on the 2-step.
On a large cube V8, it is difficult to to control the turbo speed at launch rpm with just a wastegate. If the 2 step were set at 5000 rpm and the wastegate was wide open, the boost would still have a tendency to run away above setpoint SO you use a 2-way solenoid supplied by exh gas or Co2 connected to the top and bottom of the BOV. The TBrake activates the solenoid and the BOV is pushed open,when the brake is released the solenoid switches back and spring pressure AND supply pressure hold it closed.