The feature is called MobileTrack and it uses the Android Sensor Hub, a dedicated processor that connects directly to the camera's sensors, to quietly track movement while sipping a low amount of power. Its activity-recognition algorithms can detect when you're embarking on a run or hopping on your bike without needing to be phone to be woken. Granted, it won't be able to give you the best overview of your fitness, but if you're keen to compare steps with your Fitbit-wearing friends and family, it will get you off a decent start.