to detect knock, a piezoelectric knock sensor (basically a microphone) bolted to the engine block responds to unique frequencies caused by engine knock. the sensor generates a small voltage that is sent to the electronic control unit, which processes the signal to determine if, in fact, knock is occurring. if it is, then the control unit activates a solenoid valve that directs boost pressure to the turbocharger's pneumatically controlled wastegate actuator, which opens the wastegate to bypass exhaust gases from the turbocharger directly to the exhaust pipe, lowering turbo boost pressure until the knock subsides. knock events that are managed by the apc can be "seen" when the in-dash boost needle "twitches" slightly. the apc unit has a 'knock' output where an led may be connected. this led will then light up if knock is detected. because the knock sensor becomes less accurate at high revolutions, the apc tapers maximum boost pressure after approximately 4,500 rpm.