[nkaminar]

Warning: engineering content. The field poles have a certain amount of residual magnetism. This is enough to start the generating process which then supplies the field winding and produces the required magnetic flux. If the field poles loose their residual magnetism, perhaps by sitting for a while, or are the wrong polarity, then the generator needs to be re polarized which restores the residual magnetism. In generators with an external field connection, supplied by the voltage regulator, the field is briefly connected to the battery. For a generator with a third brush the same thing can be done by putting a piece of paper between the third brush and the commutator and zapping the third brush with battery voltage. Or it can be done by using the process shown in the video in Post #6.

A word of caution: On some generators the field is shorted to ground to regulate the voltage or if there is a third brush it can be the opposite polarity of the battery (ground potential).