DRUPS. Flywheel UPS.Always running. The idea is when your main power kicks out, the flywheel keeps the power running and then the diesel generator kicks in to takeover the load. This means they have to have loads of them so they can afford to have a couple out of service at any given time for repairs (they take a fair bit more maintenance than battery systems, but seem to be preferred because of higher current output.
Wikipedia has a couple of good(ish) articles on the subject.
Biting the hand that feeds IT
