For the first time a Power Reserve energy-saving mode was implemented in Apple Watch. It allows you to extend battery life at the expense of some functions.
If you turn on the Power Reserve mode the time will be displayed, but almost any other Watch function will be turned off. The only thing you can do in Power Reserve mode is taking screenshots. You will need to press the side button to see the current time, since the watch does not react to wrist movements in Power Reserve mode.
How to enable or disable Power Reserve on Apple Watch.
There are a few ways to enable Power Reserve on your Apple Watch. For example, you may swipe up on the watch face to access Power Reserve. Another option is pressing and holding the side button until the power menu appears, then simply pressing the Power Reserve button.
To disable the Power Reserve mode and return to the usual power mode, press and hold the side button of the device until the Apple logo appears. Apple Watch will restart in a regular mode. You can only resume normal operations if there is enough charge remaining on the battery to power the device upon restart.