Lemme throw some knowledge on ya then.

I'm not sure about the OBD-1/2 cars. But OBD-0 cars(like his 89 teg) have a single LED that blips flashes, and can be seen by looking through a small window on the cover of the ECU. You count off the number of blips to find your code. Also the OBD-O ECU's will throw a CEL/MIL when they are ACTIVELY ignoring or detecting a problem. What this means is that if your "check engine" light comes on, it means the ECU has detected a problem and is letting you know(duh). Some codes can be cleared by simply swithing the car on and off, others you'll need to track down(like a dissconnected sensor). However the ECU stores the "code" for the problem it detected in memory. Thus whenever you look through the window on the ECU to check your codes you are seeing a History of all the "codes" the ECU has throw since it's last reset or loss of power.

Now at this point you are prolly asking WTF this has to do with the "timing jumper". Well that jumper that you refer to has a dual purpose. It's primary purpose is to allow you to adjust your timing via the distributor. With the engine warmed up and the jumper in place you are able to adjust your base timing advance or retarded from it's stock setting. If you try to turn the dist. w/o this plug jumpered it won't affect your timing at all.

Now the secondary effect of jumpering that plug is that it forces the ECU to clear all codes from it's memory. Most people will tell you to clear the codes in your ECU, and wait till it throws the code again. This allows you to troubleshoot the correct code without chasing after previous codes that are stored in the history. The reason I called it the "timing jumper" is becuase that's what it's main function is. HTH