You can see all the device functions by tapping the Mode button and scrolling through the different screens.
Tap ‘Mode’ button until you see the exercise mode screen:
In exercise mode, press and hold the mode button to start tracking your exercise workout. This displays your heart rate, step count and elapsed exercise time. Tap the mode button to pause / resume your exercise and hold the mode button for 3 seconds to end exercise tracking. If you do not exit exercise mode, it will auto-quit after 5 hours.