ShlomoZalman, in the above movie, they have a lot of footage of his playing, and also an interview of him now, which he rarely does.
You can see that aside from his overpowering speed, he had such control of the ball, and kept it wrapped in his hands for an extra few milliseconds or so to be able to guide it exactly where he wanted it.
He would whiff greats like Mickey Mantle effortlessly. He had a problem which he tried to work on that his pitches would be obvious to the hitters based on his windup. Another great hitter of the time said, we always knew what he was about to throw, but we still couldn’t hit it.
However, I was reading some info on him, and in the beginning of his career, he had such problems with control that he was about to give up baseball after a year or two in the majors, he got so frustrated. He decided to give it one more chance, and worked very hard on getting into the best shape he could, and practicing his throwing, and it paid off. It is a shame that he threw out his arm at an early age and was left with arthritis that ended his career.