"The knock sensor registers the sound of the engine and converts it into electrical pulses. The engine control unit cross-checks the measured pulses against the stored specification values and intervenes to control the engine, fuel injection and ignition as necessary, before the combustion reaches the knock limit."
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http://www.volkswagen.co.uk/technology/glossary/knock-sensor
By removing the mounts and going back to stock they are just treating the symptom and not the actual problem. If they were getting a lot of false knocks, they should investigate what is pinging or vibrating that shouldn't be (which would be amplified with the stiffer mounts) .