You know how an alternator works right?
Any way, with out power to run throught the rotors to create a magnetic field, there it no power being made. The alt can make more or less power by regulating the voltage to the rotors, thus placing more load on the alt, and the engine.
Once the car is started it will run fine as the alt is making enough power to keep it self active I suppose. One it stops though, with out an external sorce to start it, it's dead.