[RichardJ]

>>"flash the field"<<

An interesting term to use when you are talking about an AC Alternator.
If we were talking about a DC Generator that term would make more sense. When discussing a DC Generator, someone will always mention Polarizing the Generator.

In the first image below there are Iron Pole Pieces used in DC generators, The Field Windings are wrapped around these Pole Pieces.
These Iron Pole Pieces retain a great deal of magnetism. The Polarity of the magnetism retained in these pole pieces is crucial to the correct operation of a DC Generator.
When working on a DC Generator, it is common for the polarity of the residual magnetism to be reversed.

It is then very important to correctly align the magnetic polarity of the pole pieces, with the Polarization procedure.

AC Alternators do retain some residual magnetism, but the polarity is not an issue.

In the second image, the ignition switch connects 12V from the battery, through the voltage regulator, directly to the field winding. Full Field magnetism is generated and current begins to flow in the Stator Windings. This ignition circuit is called a Exciter Circuit.

Full Field magnetism is generated the moment the key is turned ON and also at idle. Field magnetism is reduced by the voltage regulator, as max voltage is reached.