When trying to load new firmware files, the running firmware might freeze, making making the on/off button not responsive. Turning off the watch and restarting by pressing the power button seems to do nothing.
The power button (on the top left of the watch) is connected to the PFN1 input of the MAX20303 PMIC. A 12seconds press of PFN1 will hard power-down the system. Normally, the firmware shortens this time, so a short press will send a power off command to the PMIC, but if the MCU locks up, the long press is needed.