Knock Muffs?
 

Stepson got his Nissan GTIiR rolling roaded the other week and his APEX computer now says the engine is knocking/detonation in certain conditions (140mph + full throttle :D )

To give the company thier due they have told him to bring the car back and they'll sort it. They said they had checked for knock/detonation using knock muffs

So the question is what are knock muffs and how do they do what they do?

Tris