The Drive Box chip contains one control module for the immobilizer, central locking, and alarm units. The DME (digital motor electronics) or the ECM (engine control module) which are the same thing is mounted in the trunk.
Ok, so what I gather...
The transponder in the key somehow talks to the immobilizer who is in constant communication with the DME. The DME is fully capable of doing all the heavy thinking for the car. It drives the engine under all conditions but just like Tina Turner in Road Warrior (Beyond Thunderdome), it (the DME) wont lift a finger without permission from the midget (the immobilizer) riding around on the big dude's back.
Also, apparently there's a whole frunking list of bizarre trips that can give the immobilizer all the excuse it needs to refuse to start. Things like leaving the key in the lock too long without actually starting your car. Sitting in the drivers seat for too long without starting your car. Opening the driver's door and sitting in the driver's seat with out first facing the Fatherland and raising a single arm salute. And many others...
For your enjoyment and to remind you who runs Bartertown (aka who owns your car - hint
NOT YOU):
Who runs Bartertown? - YouTube