electronic timing

i dont understand how it is possible to advance/retard timing through a chipped ecu... bottom line, if the cam is exactly set a 0 degrees, the spark plug will fire according to the position of the cam/crank, not the computer.
am i right on this? if not please explain...
ignition timing can be advanced or retarded through the computer. the cam timing, which is entirely different can only be changed through the cam gear
On an ignition with a Distributor the rotor has a contact that is about 90 degrees long. The computer can choose to fire the spark as long as that contact is "touching" a point on the cap. So your computer decides, based on the info it is getting from its sensors, when to fire the spark.