thank you kind sir for the explanation, some more question if yu would be so kind

it's basicly a mic on the engine?
almost like the old school "stick a screwdriver in and to your ear" sort of device yes?
so how do det cans work then?
are they actual cans that youput on your ears (like it's being implied in some posts) or ar they actual bits that do measuring the same way as the knock link?