How does residual magnetism help a shunt-excited DC generator begin producing voltage?

In a shunt-excited DC generator, the process of voltage buildup relies on the presence of residual magnetism within the iron poles of the machine.1

As the generator is brought up to its operating speed, this residual magnetic flux interacts with the rotating armature coils to induce a small initial voltage. Because the field coils are connected to the armature output, this small voltage drives a current through the field windings. This field current, in turn, increases the magnetic flux, which further raises the generated voltage. This self-reinforcing cycle continues until the magnetic saturation of the iron limits the voltage to a stable operating level.1