The kill switch wire (white/black usually) has to be grounded to kill the spark. If you left this wire hanging or snipped it, the engine would fire and run. If the kill switch wire is connected to the engine ground, the CDI would not spark and the engine would not run.
[edit] There is voltage on this wire when the engine is running. Enough to give you a good zap.
[edit2] If you are new to karting and plan on having these engines around I would suggest ordering at least one spare CDI, voltage regulator, and coil. I've had all three fail at some point. Swapping a new one in (After verifying wiring so you don't burn up your spare as well!) makes troubleshooting a lot easier.