Opposite of positive knock.
The ECU table for spark can be pre-set by the tuner, most target 25-26 degrees spark to be on the safe side at WOT, of course the ignition advance varies greatly depending on RPM, throttle input, load etc. etc, all best left to the tuner.
The ECU can then remove or add upto 8 degrees either way from the knock sensor reading, positive knock means it pulls timing and ideally you want to avoid this happening at all, though +1-2 degrees of positive knock won't cause harm and the ECU will retard the timing.
Negative knock is the opposite, the knock sensor lets the ECU know it can add more timing/spark which when it detects negative knock it will do.
Basically the ECU like most other modern cars can add and pull timing based on numerous sensors throughout the car, so having good fuel means typically you see negative knock and the car adds timing and the opposite happens for poor fuel. In short put in crap fuel, positive knock goes off, timing gets pulled, or engine goes boom if the car is mapped for a higher RON fuel. Put in really good fuel and ECU can see this from negative knock and it adds timing. Of course other things can cause knock sensor to go off positive or negative as well.
If the car was custom tuned they would set the spark to a point where there is no positive knock and around just a couple of degrees of negative knock. Reason is even if the car detects negative knock of say -6.00, the car will only add 1-2 degrees of spark so could still be down on power slightly. The car will pull a lot of timing if it detects positive knock (safety) but will only add in a little when it detects negative knock, again safety.
But in short negative knock just means the car is adding timing and the tune could be further optimised, in short it is safe. I am not interested in fine tuning right now, best case another 10 horsepower and a few datalogs done, right now it drives fine I am not bothered about 5-10 horses, so shall leave. Also the ECU learns over long term, in short keep putting in good fuel and it will eventually just set more ignition itself, but will take a few tank fulls to get there. Fill with crap and it will pull that timing real quick but will take a while to put it back.
They can get the tune optimal once manifolds are fitted, I prefer to play it safe.
P.S. The Ford stock map also has negative knock when filling with Vpower, as the car adjust power depending on octane.
