From the version 1.8, you can double tap at any time (except in the menu and the fuel page) the watchface screen to start/stop/reset the chronometer (the thin red hand).
The chronometer is not blocked if you enter the menu or of you change the theme.
When the screen is ON, the watch will vibrate every minute passed since the chronometer started. When the screen is OFF (power saving mode), no vibrations will be generated but the chronometer runs in background.