The fact that he did not turn his wrist at all is what made the ball spin viciously in the air "without losing the integrity of the seam position".
We have seen this many times from modern spinners. For example Anil Kumble's top spinner is a violently spun ball with seam up right. But it defies physics how a forward spun
ball instead of back spin
, produce swing. It produces drift and dip, but I am not sure about the swing. Any further details on this?