It was supposed to reduce the drive-by noise which would have been a problem with the regulators on the Trophy. What happened was that the early Trophy (Vin nos from YD525704 to YD530688) was fitted with a standard exhaust at the factory (to strictly meet regs), shipped to the dealers who then fitted the Trophy exhaust plus the vacuum activated air filter box valve. Fairly shortly after with the MEMS 3, they fitted a more complex ECU driven vacuum system with the exhaust flap.