[yetibone]

Actually Richard, that's pretty accurate. It is a very complex system, built around a simple idea. Ionized gasses are conductive, or can carry electrons, much like lightning jumping from cloud to cloud during a thunderstorm. When a low voltage load, roughly 80 to 100 volts is applied to the end of the spark plug, it will strike an arc, so to speak, in a heavily ionized environment.

Instead of that current jumping to the spark plug's ground electrode, it finds an easy path to ground thru ionized combustible gasses in the chamber. The ECM can then detect a drop in voltage at the tip of the plug by monitoring how much current it takes to maintain that 80 to 100 volts.

The higher the current needed to maintain 80 to 100 volts, the greater the drop in voltage. The greater the drop in voltage, the greater the number of ions present in the combustion chamber. By monitoring the current (amps, not volts) the ECM can calculate how long to delay the firing event.

Ionization of a combustible charge is due to heat, which is obviously a byproduct of combustion itself, but as that charge is pulled in and compressed
into 1/10th it's original mass at 14.7psi (pressure @ sea level) it begins to superheat. That's when ions start gathering together.

An ion sensing knock retard system is superior to a piezo-electric knock sensor in that it can detect the knock before it happens. A piezo-electric knock sensor senses the knock itself, and retards the ign timing after spark knock has already occurred.