FWIT the voltage is constant (9vdc +/-) unless the knock sensor detects knock and then the voltage goes to zero.
Then when the knock is gone it goes back to 9vdc.
This voltage/no voltage occures many times per second when knock is present.
The amount of time that the voltage isn't present is how the computer determines how much timing to pull.
This is how I understand it works (Can be wrong).
I don't know what values the Casper Knock Gauge works off of.