"Smartwatch mode" refers to a power-saving state on a smartwatch where certain features are turned off or limited to extend the battery life. Full definition
It also features a battery life of up to seven days in smartwatch mode. (pocket-lint.com)
Its battery life goes up to five days in smartwatch mode and two weeks in standard watch mode. (pocket-lint.com)
In addition to being water - resistant up to 50 meters, the Forerunner 645 Music has an estimated battery life of seven days in smartwatch mode or five hours in GPS and music mode. (arstechnica.com)