Power reserve mode, not mechanical.

>When the battery level reaches 10%, you will receive a notification. At 10% you can no longer run a Respiratory Scan. When the battery reaches 0%, your watch activates power reserve mode. Only the analog clock, step, and sleep monitoring features remain available, and only for about a few days. In power reserve mode you will no longer be able to start an ECG or oxygen saturation measurement on demand via the watch display.

It only happened once or twice, I was surprised that once you charge you still see the step tracking stats when it syncs. I remember using it in this mode for over a week when I was moving and couldn't be bothered to find the charger.