The way I read it, it is the magnetic field which rotates by electronic switching off three phase current, much the same as the trains I drive. AC three phase motors/generators work differently to DC in that they need thyristor control to switch current on/off during the wave form which, due to the oval stator, is simulated. If you read it, nowhere does is state that the rotor is moving. Torque can be measured without movement, simply apply a socket or spanner to a very tight bolt and try to undo it, even when it won't move, you're still applying torque, just not enough.