what the thing does is that it'll rotate the air going into the plenum.. so when you release the throttle, the air can maintain in the plenum becuase if the air needs to exit back through the surbo, it need to change its rotation and due to the air's inertia, that is impossible unless there is outside force to change it. thus theoritically it'll maintain slight pressure in the plenum. when you open the throttle again, you don't need to wait for the air to gain momentum again thus improve throttle responds.
BUT, since air is very light and can easily alter it's velocity and trajectory, it cannot maintain the "pressure" for long (long meaning less than 2 seconds). so the improvements can be felt during gearchange and during attacking cornerings .etc.. if you close the butterfly for too long the air will lose it's momentum again. if you stop at the traffic light or going downhill for a long time, you won't see or feel the difference.
better option? get a larger plenum. larger plenum can improve car's performance as it'll also maintain the pressure/air longer.