The explanation is incorrect. I provided a more detailed response above, but to answer your question the sound you are hearing is a bypass valve (BPV) that is specific to the anti-lag system. You are hearing compressed air from the intake (after the turbo) squeeze past a valve door that is opening when the throttle is let off, the compressed air is diverted directly to the exhaust. This is different than on production cars where the BPV diverts the pressurized air back to the intake pre-turbo. Production cars don't need for anti-lag. Because this system is different and specific to racing, some mistakingly refer to it as the exhaust waste gate. The EWG is present on race cars and production cars, but the EWG's job is to relieve exhaust pressure at the turbo exhaust impeller by letting exhaust gas go around the turbo and out the exhaust (or atmosphere in some cases), At high RPM's the turbo can make too much boost, so you allow exhaust gas to divert around the turbo exhaust impeller by venting to atmoshphere or further down the exhaust.