In reply to frenchyd :
The thing is, though, it does take power to drive the compressor, which comes from exhaust energy via the turbine. With nothing driving the turbine off throttle, the compressor slows down. Turbo speed is boost, not so much actual flow (within limits: see the speed lines on a compressor chart)
An RX-7club member logged turbo speed with and without a blowoff valve. The turbo actually slowed down a little faster with a blowoff valve, because the turbo is under more load if it is compressing air vs. if flow is stalled. Stick your hand over a hair drier and hear the motor spin faster because you've removed the load...
