As a curveball flies through the air, the spin creates an imbalance of air pressure on either side of the ball. According to Bernoulli's principle, this imbalance of air pressure creates lift. When applied to a spinning object, the Magnus Effect holds that the force of lift generated will cause the spinning ball to move in the direction of lower pressure.
To throw a curveball, start by holding the ball between your thumb and middle finger. Then, as you wind up and throw the ball, snap it on the release by rotating your thumb and middle finger down, almost like you're trying to snap with them. Try to release the ball as close to your body as possible, which will make it spin and curve more. Don't worry if the ball doesn't curve at first. Keep practicing the snapping motion when you release the ball and over time you'll get better!

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Recommendations: Drills to address “good arm action” should focus on getting both arms working together in concert. What the glove arm does directly affects the throwing arm and there should be a sort of seesaw effect. Establish the positions, but practice moving right through those positions in a fluid, efficient manner. And always remember, every pitcher is different, so let young pitchers find their own natural arm slot – avoid teaching cookie cutter pitching mechanics.

The Curveball is generally a more advanced pitch to learn and throw because of the unique mechanics involved. Many coaches feel that younger players (below the age of 15) should not practice throwing the Curveball with real baseballs because it might place too much stress on a young arm. Although it is likely much safer to throw curveballs with a Blitzball because of its light weight, younger players should still ask parents' for their approval before trying to learn it (you might want to learn a hard Knuckleball instead to get a similar dropping action without having to throw actual Curveballs). The Curveball is unusual because it is the only pitch that involves "topspin" (think of it as the opposite of a fastball's backspin).

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.
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I would recommend waiting until the player is 14 or 15 years old. If young players throw curveballs on a consistent basis at younger ages they can cause damage to their elbows and thus hinder the growth process. But it’s not only the fact that they are throwing curveballs at a young age, it’s the fact that they are throwing curveballs with improper mechanics that causes much of the damage. The key is to make sure they are throwing the curveball with proper pitching mechanics.
In this video I share with you how to make a tool that you can use to work on your curve.  What you do is screw two baseballs together.  For the pitching drill you want to throw the balls in a way that they rotate tightly end over end on the axis in which you are trying to get your break.  This tool and pitching drill can dramatically improve your curveball.