Under normal circumstances ...
1) Key on, engine off, no jumper, no cranking - CEL should be on, goes off after engine start.
2) Key on, engine off, jumper in, no cranking - CEL should be flashing.
If the CEL remains on after engine start, there is either a code set or the "service engine soon" (SES) system has flipped over - this is when you insert the jumper and count the flashes as in #2 above. If the CEL stays on rather than flashing, you reset the SES by flipping the slide switch, and then the CEL should flash, count them, if it's a code 12 (no problem detected), pull the jumper, service the engine and continue.
From what you say, it seems like the only way you get the CEL on is with the switch - this suggests either a defective ECU or an ECU wiring related issue.