Wall Drill by Norm Chadwick
Stand as you would to begin your windmill motion, but position your self with your throwing shoulder against the wall. Start your windmill motion and as your hands go forward and up pivot your body into an open position so your body faces the wall. Both of your hands should be above your head and in contact with the wall.
Now continue your motion and try to keep your hands in contact with the wall through the entire motion. The wall represents the pitching plane that your arm circle should follow through your entire motion. Having your front side hand follow the pitching plane as well keeps your arm circle following the correct pitching plane. You want your body to be in the open position at release.
To paraphrase Michele Smith, the glove side leads and the throwing side follows. Your glove side is your dominant side; if it leaves the pitching plane too soon it can pull the throwing side off the pitching plane. Following this pitching plane helps your right to left control. Moving your pitching plane slightly, adjusting your front shoulder position a little left or right, helps you hit the inside or outside corner.
Norm Chadwick has 12 years of experience in providing private and group lessons to all ages and skill levels. He teaches basic mechanics, all types of breaking and off-speed pitches, strategy, and game preparation.