Different speeds via capacitor changes is for multi fixed speed fans, not variable speed via dimmer.
Friction control? In a very indirect way, I suppose. The witnessed phenomenon of the fan rotor speed increasing if the fan blades are removed is explained below.
Torque control? Nothing that sophisticated, but that is the net effect yes.
The dimmer works to reduce the net average VOLTAGE going to the motor. That in turn reduces the torque that the motor can produce by the square of the voltage reduction. So at 50% voltage, the motor produces a maximum of 25% of rated torque. Then because the fan blade pitch is fixed and the air density doesn't really change, there is an EFFECTIVE change in the speed of the fan, because the reduction in torque capacity in the motor means it increases the "slip" of the motor. Slip is the difference between the speed of the magnetic field rotation (determined by the line frequency) and the actual speed of the rotor; higher slip means a greater difference, ergo slower rotor (blade) speed. Slower blade speed = less air movement, air movement = load, load x time = energy, so lower energy use.
If you remove the fan blades, the rotor speed increases because there is no no load on that motor other than it's own mass, so it is not doing any useful work.