This switch represents the check engine light. The power source would be the cars cdi signal to the light. The current source is the alternator. With a pnp transistor in line with the base connected with the alternator and the collector and emitter bypassing the the original circuit path, rerouting the current to light when powered by the key "on". When turned to "start", the current triggers the transistor and stops the ability to send power to the light. This is to fool emission checks in states that require your check engine light to pass for smog examination.
