An electric generator or electric motor that uses field coils rather than permanent magnets will require a current flow to be present in the field coils for the device to be able to work. If the field coils are not powered, the rotor in a generator can spin without producing any usable electrical energy, while the rotor of a motor may not spin at all.
For older and very large power generating equipment, it has been traditionally necessary for a small separate exciter generator to be operated in conjunction with the main power generator. This is a small permanent-magnet or battery-excited generator which produces the initial current flow necessary for the larger generator field to function.
Modern generators with field coils are self-excited, where some of the power output from the rotor is used to power the field coils. The rotor iron retains a residual magnetism when the generator is turned off. The generator is started with no load connected; the initial weak field creates a weak voltage in the stator coils, which in turn increases the field current, until the machine "builds up" to full voltage.
Self-excited generators must be started without any external load attached. A external load will continuously drain off the buildup voltage and prevent the generator from reaching its proper operating voltage. Because the self-excitation voltage starts out much lower than normal, an external load is capable of absorbing many times its normal current rating due to the Volts x Amps = Watts law. A 240 volt, 5 amp load appears to be a 240 amp load when driven with 5 volts, which is essentially a dead-short to a self-exciting generator trying to start up.
If the machine does not have enough residual magnetism to build up to full voltage, usually provision is made to inject current into the rotor from another source. This may be a battery, a house unit providing direct current, or rectified current from some other source of AC power. Since this initial current is required for a very short time, it is called "field flashing". Even small portable generator sets may occasionally need field flashing to restart, using procedures documented by the manufacturer.