it is there to detect any unwanted noise being generated by any type of mismatched event,i.e, preignition/auto ignition n that might be leading to detonation.
So in order to avoid detonation the ecu needs a sensor which can hear the engine knock sound.
Besides few days back i have done this kind of work on a y8. Believe me bro i had to do that fabrication.Without the knock sensor, the ecu was not switching over to the higher power cam profile. Waysay,,! y8 head only switch over from 12v to 16v configuration(vtec-e), so no high lift profile involved here.
In another (dual vtec) instance, which i m doing these days, the ecu is not switching over to the second vtec. The reason, because the ecu is not getting the proper signal from the knock sensor. And the reason of receving wrong signal from knock sensor,,,,is,,, that one of the pin of a piston has become loose inside the piston n so generating a "tick tick' sound.
any thing else?