Good question sir - please see following explanation. (this is for the Spits. Not sure about Hurricane)
Let's assume the throttle slider is at 100% to begin with.
OFFLINE:
Depress the boost cut-out key
Nothing happens at first - need to jiggle the throttle to activate boost (maybe there's a bug where it doesn't activate until an input on the throttle is registered)
Boost dial increases from 6 up to 8. Noticeable change in pitch from engine - speed increases correspondingly.
ONLINE:
Depress the boost cut-out key
Nothing happens - does not matter if I wiggle the throttle or not. No change in the boost dial and no change in engine pitch or speed.
I would post a little video, but I can't get Fraps to work since the last patch. If you need evidence, I'll film it on my camera phone and post it up on YT.
I would have done it tonight for you, but it's pretty late and I've been out for beers.... hic