ESR's outline is more-or-less correct. Electric current is required to maintain a magnetic field. When the field collapses some mechanical device takes over u

ESR's outline is more-or-less correct. Electric current is required to maintain a magnetic field. When the field collapses some mechanical device takes over using stored energy (springs and/or gravity) Here's a practical example to give some more context to the idea: https://patents.google.com/patent/US3721462A/en https://patents.google.com/patent/US3721462A/en https://patents.google.com/patent/US8811561B2/en https://patents.google.com/patent/US8811561B2/en