Random countdown to Power Save for no apparent reason

Every now and then, even when the battery is full, my Forerunner 945 LTE will buzz on my wrist and I'll look to see why and it's saying "Power Save" and there's a countdown ring slowly counting down to a moment when, presumably, something is going to happen related to something called Power Save that I don't even know what it is. I think it's fair to assume it has something to do with saving battery power but when this happens while batter is above 95% it's truly strange. Would love to know what's going on here.