The DC motor has four available terminals, which are two for the field coils (terminals 3 and 4) and two for the armature coils (terminals 1 and 2). In a small number of low-power electric motors, the field coils are replaced by permanent magnets. In this situation the motor has only two access terminals (terminals 1 and 2). https://www.mrosupply.com/hydraulics-and-pneumatics/fittings-and-connectors/shank-waterfittings/113212_rss35_dixon/ The elementary modus operandi of a DC motor is based on the mechanical force acting on a conductor inserted in a magnetic field, when an electric current flows through it.
In the coil 1, the forces are similar and opposite, generating no rotational force (torque or torque), however the coils 2, 3 and 4 have on them a torque Fx so as to urge the rotor to rotate, coil 1, which then enters the region (of coil 2) where the coil 2 was, and then starts to operate a turning force as well. For this scheme to operate, it is fundamental to modify the current direction of the armature every 180 °. The component that switches current direction is the switch.