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It's a switch that connects several resistances, one at a time, to a motor to allow the motor to start slowly. The resistances are switched as individual resistances from the highest to the lowest. As each resistance is switched in, the motor receives a certain amount of current, as the motor reaches the speed that switch setting would allow, the next resistance is switched in, replacing the first resistance.
The motor speeds up a little more, until zero resistance is reached and the motor is running at full speed. Best advantage, it is inexpensive, and is good for relatively small motors. Disadvantages, arcing between contacts, motor speed up is not a smooth transition in speed. The shock of each speed increase can cause damage to the motor, or it's load.