I asked a smart person the following question "So I am stopped at stoplight, and the car starts rolling backwards. I press on the accelerator to stop the rolling. What is happening?" He said that the drive starts rotation the stator field at some slow RPM. That rotating field induces voltage in the bars in the induction motor. The resulting current produces a field that interacts with the rotating field, producing torque to retard / stop the car from rolling. I guess the same thing can be said about regen braking, since it is good to almost zero speed.