Have a look in knowledge base, there is an excellent article in there, written by one of our members, Tony, who has a wealth of knowledge on this subject, you will also find some very good drawings too.
Something I should explain, the difference between “slip” and “slipring”.
Slipring is easy, it’s the rotating connection between the rotor and the stationary current control “rheostat” as you call it.
Slip is slightly more complicated. When the current is applied to the stator the rotating magnetic field is at a fixed speed, the rotor has to catch up. The rotor current is controlled by the starter resistance and gradually brings the rotor up to speed controlling the motor torque. Immediate full on torque would probably rip the machine apart, the effect on the distribution system would be catastrophic.