The ECU even in the older cars has a complete timing map just like fuel. When the distributor is set to 16° BTDC on an OBD-I or older car, the actual timing map that's in the ECU is what happens. If you advance to say 18° BTDC, all you're really doing is adding 2° to every point on the map. That's on the non-OBD and OBD-I cars.

On OBD-II ('96 or newer) cars the ECU is able to detect that it's not getting the proper timing and "learns" to apply whatever sort of adjustment factor to get the values that are on the timing map. So if you advance the timing by a couple degrees, that advance will only last for a week or so and then the ECU learns how to dial it out. If power is removed from the ECU, it resets back to the base map. Some people with OBD-II Preludes have rigged up a system of relays to cut power to the ECU every time the car is shut off just so they can keep their advanced ignition timing. One problem with this is that if the ECU throws a code there's no way to check it since the memory is erased once the car gets turned off. It's not really worth the effort just to advance the timing by a couple degrees.