PC is a piggyback system. It doesn’t flow anything to the ECM. It takes the stock signal in the ECM and makes the necessary adjustments to match the output from the tune you select to all the parts downstream like injectors ignition sensors etc.
I can't say how all Power Commanders work, but I know that is not exactly how mine works. The tables I load into my PCIII for fuel and timing ARE the adjustments, they are not a target. This is why it's important to know whether your ECM has been flashed with a "stage 1" or "stage 2" calibration. This is why when you look through the list of canned maps for the PCIII, you will see that they specify whether they are meant for a stock ECM or one that has been flashed.
For example, if you look at a fuel map and a particular cell contains the number 10, that means when the conditions of the cell are met (rpm and throttle position) whatever the duty cycle is that is transmitted from the ECM to the injector will be increased by 10% (% referring to fuel flow), meaning that if the ECM says to spray fuel at 20% the PC changes that to 30%. Likewise with timing, a number 4 in a cell will advance the timing by 4 degrees from whatever the ECM says.
This is also why a map that works well on my bike may not work well on yours even if we have identical builds. If our ECM are flashed differently then the same maps will produce different results.
Of course, if you are tuning on a dyno then the calibration of the ECM doesn't matter because you are tuning to external sensors.
Lukas · updated 
