What is a rotor position sensor?

Also called a hall sensor. It watches the magnets on the rotor go past and reports speed and direction back to the control board. If that signal disappears or goes erratic under load, the board assumes the motor is jammed and throws a locked motor code (LE on LG, 3C on Samsung) even though the drum spins freely by hand. On many newer machines it is not sold as a separate part at all, it is integrated into the stator assembly, so the repair means replacing the stator.