for those that want to know.
a knock sensor detects detonation/pinking etc in you cylinders (caused by bad timing and bad fuel ie: low ron rating)
a knock sensor works by listening to the sound in your cyliners, basicly its a sensor that works within a specific frequency range that when it detects a specific sound (ie: pinking/detonation) it sends a signal to your ecu which either retards or advances you timeing to keep the engine running safe.
for those tuning hondata with cars fitted with knock sensors will find the knock warning light will spike on a data log when vtec engages, (its the pins locking the vtec proflie into place). dont try to tune it out just ignore it - if it spikes anywhere else then worry like mad.
cheers