3. Elbow: The throwing elbow must be equal to or slightly above the throwing shoulder. As soon as the pitcher lowers the elbow below the shoulder, they put additional stress on that arm. The angle of the elbow joint should be no more than 90 degrees. Pitchers who throw curveballs at angles greater than 90 degrees may put additional stress on their throwing shoulder.

6. Arm speed: It is extremely important that you maintain similar arm speed with your curveball that you have with your fastball. A hitter reads arm speed. The matter of arm speed is obviously more important with a changeup than it is with a curveball, but it’s important for other reasons. Another way to get a tight rotation and hard downward movement with a curveball is to throw it with quicker arm speed. If two pitchers have the exact same curveball grip, mechanics, release, etc. but one throws it with quicker arm speed, the one with quicker arm speed will throw the curveball with more break, and thus the harder pitch to hit.
One of the biggest issues pitchers have when they begin throwing a curve is changing their fastball mechanics. Don’t! Throw the curveball, or any other pitch, using the same arm slot and arm speed as your fastball. The only thing that changes is wrist and forearm angle. With the curveball your wrist and forearm angle look like a “karate chop”. To get an effective rotation on the ball, released the ball late. The curve will squirt or hang when you release it early or you don’t keep your glove in front of you at release. I explain this in greater detail in the you tube video below.
Certain relief pitchers specialize in this type of slider. Known as “Lefty One Out GuYs” (LOOGYs) or “Righty One Out GuYs” (ROOGYs), they are brought into a game to face a dangerous same-handed batter. They usually feed the opponent a steady diet of these “runaway” sliders, tempting the batter to swing at a ball that looks like a strike – but isn’t.
Naturally, when you do the 1st two steps correctly, this will follow suit.  Downhill means more tilt or depth.  A 2-plane Slider which breaks Down & Away, or Down & In off the plate, does far more damage to hitters.  Its not a side to side breaking pitch.  When you throw the Slider Downhill, hitters will say that they can’t see the dot.  Their frustration builds as they cannot pick up in the spin.  That is because the dot in on the bottom of the baseball.  Needless to say, this is when you will miss more bats.  Obviously, it means MORE STRIKEOUTS & A LOT OF PISSED OFF HITTERS WHO FACE YOU!
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In order to master the curveball, you must start by gripping the ball with your middle and index fingers together, with the fingers across the seams of the ball at the widest part (the widest distance between the seams). Keeping a tight grip on the ball, especially with the middle finger, don’t let the ball touch the palm of your hand, or you won’t generate enough topspin, which is what allows the ball to drop when it gets close to home plate.
In this video I answer some questions about the King of the Hill Pitcher’s Trainer, one of which was “what type of pitching drills can you do on the King of the Hill?”.  If you like the King of the Hill, and I definitely suggest this product for pitchers of all ages, then you can check it out at King of the Hill.  I really love this product because it teaches pitchers how to properly and efficiently get the most energy out of their legs.