What you are refering to is a tool for a mechanic to tell if he has the correct heat range on a highly modified engine and also as a method of telling the health of an engine. This is referred to a "cutting clean". It entails shutting the ignition off during moderate acceleration, when the fuel system should be "clean" (not overly rich or lean). For the most part, there are better ways to tell the same thing today. This is a throwback to the 50's.
