The knock sensor PID is measuring crank degrees in the direction of rotation that are added to or subtracted from the normal (usually borderline knock table) spark timing.
When the KS PID reads positive numbers, it means there are more crank degrees at ignition, which means a spark that happens later than normal, and most people call that ******. A negative number means fewer crank degrees at ignition; sure enough, that's ADVANCE.
When you first encounter it, it seems odd, but once you understand it, it makes sense.
You should normally see negative numbers as the KS looks for incipient knock - that's how the gets the most power out whatever octane you're running. It hunts relentlessly for the edge of knocking. Positive values mean your spark tables are providing too much advance.
In laymen's terms, this means "negative knock," is the computer adding more timing, and "positive knock" means the computer is pulling timing due to actual knock.
Taken From: How to: Understand your datalogs