I think by "accu" he meant the battery. As you noticed, there's an option to optimise battery usage in the batty settings section and you can activate it by switching the toggle button. They explain activating the battery optimization is supposed to extend your battery life by preventing apps and watch faces from transferring data when not in use. As he also commented, you may need to disable the heart rate monitor app and the stress monitor app or to set them to measure your heart rate less frequently. Then you activate the battery optimization. Hopefully, this will prevent the heart monitor sensor from being triggered constantly and without need. I'm not quite sure why you said it would go back to the automatic mode again. It's not supposed to. Try to disable the heart monitor and the stress monitor or set them to lower frequency, go back to the home screen (on your watch) and then go back to the settings to see if the settings have been saved or not. If not, you may need to restart your watch before (and after) you change the heart monitor settings.
... View more