The discontinuous variation rheostat is a rheostat that has a bank of resistors with well determined values, which guarantee the association of resistors in series, to raise the resistance value or in parallel, to reduce the resistance value. In DC electric motors the current peak is very high, so to minimize the peak of this current this component is applied as a starting rheostat so that the motor is driven gradually.
In this way, https://www.mrosupply.com/lubrication/lubrication-accessories/2310397_8072-b_alemite/ by entering the maximum resistance of the bank, the resistors being removed in stages, the electric motor will start more slowly because the resistance of the circuit will be high, the current will be low, and according to the decrease of the current resistance will increase slowly. Thus ensuring that in the first step the motor starts at a lower speed and after removal of the resistors it will raise its speed, thus decreasing the probable peak of current. It is possible to identify then that this component plays the same role as a fixed resistor, but with a greater advantage of obtaining the characteristic of modifying its resistance.