[cjlars]

Spring drive's are cool. I'll try my best fast explanation: it has an ordinary rotor and spring barrel, but replaces the escapement with a flywheel, an electromagnetic break (like you have on an ebike) and a quartz controller. The rotor winds the spring barrel, the spring barrel powers the flywheel, and the break generates electricity for the controller, which keeps track of the time and then in turn tells the break how fast to allow the flywheel to spin.

The fact that they sidestep the need for a battery (the spring barrel pretty much is the battery) is the clever part here. It's more efficient than using a rotor to charge a battery and run a stepper motor, and more accurate than a purely mechanical escapement. But it's still fundamentally a quartz watch, which turns some purists off.