I'm not a professional pitcher obviously, but I've never gained control by lowering my arm slot, I had to be over the top on delivery to be effective at all. So I ask: is this witchcraft? Some pitchers do develop better control by throwing from a lower arm slot. A big part of it (throwing motion) is just what feels comfortable, and more importantly, what allows you to repeat your mechanics and motion over and over again. I don't like throwing sidearm or throwing from a lower arm slot, but it definitely improves movement on the ball (running action). I'm trying to find a gif from @PitchingNinja, but I think you guys understand it. Throwing overhand or from a high vertical slot allows you to get more on top of the ball, and is better for throwing breaking balls (curveballs) from what I've been told.