Apple revealed how the new power-saving mode works on the Apple Watch with watchOS 9

Apple has published documentation with details about the new Low Power Mode in watchOS 9.

Here's What We Know

So, the new mode turns on automatically when the battery level drops to 10% and turns off when the battery is charged to 80%. You can turn on Low Power Mode yourself in the battery settings if you want.

Activating Low Power Mode turns off the Always On Display, high and low heart rate notifications, background heart rate measurement, background blood oxygen measurement, and workout reminders. The watch can also turn off Wi-Fi and mobile communications when you're not near your iPhone. If you activate Low Power Mode the watch will run a little slower and close apps in the background more often.

Source: Apple