The Reed switch is biased by the position of the rotor magnets. Once contacts are closed a magnet field is produced in the stator coil that advances the rotor.
Advancement of the rotor allows the Reed switch to open and the magnetic field established in the stator coil gives an inductive kick of voltage. This is bypassed by the diode in parallel with the stator coil. The purpose of this diode is to nearly eliminate the spark across the Reed switch contacts.