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The knock sensor is a specialised microphone which "listens" for the engine knocking or pinking - hopefully hearing it before you do.
The ECU adjusts the ignition timing, advancing it until the onset of pinking is detected and then backing it off a bit. This adjustment is done frequently (maybe continuously - not sure of the exact details of Trionic) so that changes in fuel grade are accommodated quickly.
If it is all working properly then you won't hear any knocking from the engine.
 

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Sorry
Whilst the above is true for ordinary cars I have just been reading about the Ioniosation Sensor in the Performance thread and it seems that Saabs don't use the microphone method - the end result is similar though.
 
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