In a headset, looking around replaces moving a mouse across a flat 3D view. Head tracking measures how your head moves so the view can follow it. section 15.2
How can a phone tell which way you turned? Its accelerometer detects movement, its gyroscope detects rotation, and its magnetometer supplies a compass direction. Sensor fusion combines their readings to determine head movement. Paired with slightly different left- and right-eye views, that movement helps create a stereo 3D experience. The sensor-fusion math and tracking code are left aside. section 15.2
