If you got below 10% it would ask if you wanted to switch to this mode. You could also turn it on in the battery settings.
I've read that in this mode you could get a week or two on a full battery.
Sometime around watchOS 9 they replaced this with "Low Power Mode" (LPM). LPM reduced things like notifications, background processing, and update frequency enough to get about 50% more life out of a non-Ultra and 100% more on an Ultra 2.
LPM is gone from watchOS now but the underlying functionality still works and there is still a way to access it. You have to turn the watch off. While it is off if you press the crown it will briefly display the time.
If you wanted to frequently switch between normal operation and this low power time-only mode it would be somewhat of a pain since you'd have to turn the watch off and on to switch modes, and watchOS boots really really slow.
If you don't have to quickly switch between modes though it might be reasonable.