If you look closely..you'll see that there's a 1-way valve inline between the booster and the intake manifold. So the brakes are good for 1 use while under boost. But as soon as the throttle snaps shut, the brake booster would be re-pressurised under full vaccuum.

So the secret is ..if you are under full boost..then don't use the brakes.. Not good for rallying, where antilag is used, I guess.

Chris