The coiled rotor induction electric motor is different from the rotor motor in squirrel cage only for rotor. The rotor is made by a laminated ferromagnetic core on which are installed the turns that make up the three-phase winding, usually in star. The three free terminals of each of the coils of the three-phase winding are connected to three collector rings. https://www.mrosupply.com/electrical/electrical-boxes-brackets/floor-box-accessories/5805996_rf151dtr_hubbell-wiring/ These three rings are connected externally to a starting rheostat, made by variable resistors, also connected in star. In this way the rotor windings are also in closed circuit.
The function of the starting rheostat, connected to the rotor windings, is to reduce the high starting currents and at the same time increase the torque, allowing heavy loads to start in the case of high power motors. As the motor accelerates, the rheostat resistance is progressively depleted until the rated motor speed is short-circuited (withdrawn) when the motor starts to run at rated speed. Upon entering the rated regime, the coiled rotor motor operates with the rotor elements shorted (such as the squirrel cage rotor motor).