This works when the brushes give power to the commutator, which is divided into three sections, so only two sections are powered at one time. The commutator gives power to the wire that is wrapped around the armature (the armature is also divided into 3 sections), thus creating an electro-magnet with two poles north and south. This is where the two magnets come in; they are placed at either side of the armature. So when two of the sections in the armature are magnetised one magnet will pull the North Pole and the other will pull the South Pole. Then as the poles change because of the rotating commutator they will push away the two poles, then pull, and then push… causing the axle (which the commutator and armature is fixed to) to spin.
