@Panicslayer
The Flame front's speed of the Natural Gas is slower then The Atomized misture of the petrol. So in order to create useful presure inside the cylender n on top of the piston it is advised to advance the ignition timing according to the requirement of the fuel. This does not mean k u cant use Cng without an Advancer.
The ignition advance process is controled through different parameter, i.e, air density, RPMs, Intake air temprature, Block temprature. From all those input the ecu selects the ignition advance degrees per increase in the RPM.
A real advancer should interrupt the input signals of the ecu n alter it to get the desired advance or should directly be contected to to the trigering sensors to register the data by itself or should alter the output signal by its own n as per the preprogramed timings.
Most of the advancer being used here in pakistan just alters the signals of the map sensor to let the computer think that there is too much vacume so that the ecu would advance the ignition timing to its max. This sort of ignition advance solve the problem upto an extent but not completly.
Our beloved cheaters, while coping forgot that the original ecu was never had an ignition advance curve for the cng.
hope that answers would help u in understanding what is y n in future u would look pakwheel as a good site.