PocketLab Voyager is in the middle of a coil of wire. The rest of the wire is also wrapped in a coil, and a magnet is used to induce a current through the coil/wire. The magnetic field from the coil around the Voyager is then measured by the Voyager's magnetometer. The right hand rule tell us the magnetic field from the coil will be along the z-axis of the PocketLab Voyager. In the video you can see the z-axis of the magnetic field graph (lower graph) change each time the magnet enters. Science is awesome! (Click here for the original tweet).