I have no particular or specialist knowledge here but wikipedia says this...
"Once the engine is running and the alternator is generating, a diode feeds the field current from the alternator main output, thus equalizing the voltage across the warning light which goes out. The wire supplying the field current is often referred to as the "exciter" wire.
The drawback of this arrangement is that
if the warning light fails or the "exciter" wire is disconnected,
no excitation current reaches the alternator field windings and so the alternator, due to low residual magnetism in the rotor will not generate any power.
However, some alternators will self-excite when the engine is revved to a certain speed. The driver may check for a faulty exciter-circuit by ensuring that the warning light is glowing with the engine stopped."
Your vehicle may be different...
Peccavi
November 2009