The key with the slider is to hold the ball slightly off-center (on the outer third of the baseball). Remember to slightly cock your wrist, but don't stiffen it. That way, you can still get good wrist-snap upon release. If your wrist is slightly cocked to the throwing hand's thumb side, your wrist-snap will enable you to have the pitch come off of the thumb-side of your index finger, which, in turn, promotes good spin on the ball.
One of the most commonly used verbal pitching cues is for an instructor  to say “chest to glove”.  The cue is designed to promote proper shoulder and trunk rotation into the release of the pitch.  Many young throwers struggle with maintaining a firm front side throughout the entire throw and when the mind decides to fire the baseball forward, the front side leaks open, and command issues arise.   … continue reading
A slider is meant to be slightly more deceptive than a curveball because it is thrown harder and has spin that more closely resembles a fastball — although it doesn't create as much overall movement. Many power relief pitchers possess only a fastball and a slider in their arsenals — with one pitch setting up the other because of the late deception created by the slider.
To start, righties should take their sign from their catcher from the right side of the rubber, lefties from the left (No. 1). Take a controlled, small step back keeping the weight of the upper body over the pivot leg (No. 2). Turn your hips to the catcher and lift your lead leg from the knee into the balance position (No. 3). Do not swing the lead leg into the balance position, it's simply a "lift."
The Shadow to Balance Drill is highly effective in getting pitchers to "learn" the all-important first stages of the pitching motion – getting from the stance to the balance position in a controlled and balanced manner. Because no baseball is used in this drill, a pitcher can practice this beneficial exercise on a daily basis, regardless of when he is pitching during a particular week.
Bruce Sutter, one of the best splitter pitchers in the history of the game, says that it is very important to put your thumb on the back seam, not the front seam. This puts the ball out front just a bit more than a fork ball. Then, he says, you just throw a fastball. A very sophisticated and misunderstood point is that the split-fingered fastball should be thrown with back spin just like a two-seam fastball. But in a Roger Kahn / Bruce Sutter interview in Kahn's book, The Head Game: Baseball Seen from the Pitcher's Mound, he points out that this is not the case.

Typically, it's only a good pitch if you've got bigger hands. That's because the pitch itself should be "choked" deep in the hand. This is how splitters get their downward movement. Your index and middle fingers should be placed on the outside of the horseshoe seam. The grip is firm. When throwing this pitch, throw the palm-side wrist of the throwing-hand directly at the target while keeping your index and middle fingers extended upward. Your wrist should remain stiff.
For example, physical forces and properties, such as gravity, friction, velocity, acceleration, and momentum, are constantly at work when a ball is thrown and subsequently hit by a bat. When it comes to a curveball, though, the spin put on the ball by the pitcher brings a couple of other scientific principles into play: Bernoulli's principle and the Magnus Effect.
When you start actually throwing the curveball, I recommend getting the feel for ball release first and working backwards from there. So start off real light, about 15-20 feet from your target. I usually like to start guys off facing their target, feet shoulder width apart, toes pointed forward. You can even start out on your knees to take the legs out of it completely (forces you to keep it light). And just get the arm up with that good hand and wrist position, and work on tossing/flipping the ball with good forward spin.
Why does a curveball curve? How does a pitcher throw a changeup with the same arm speed as a fastball? Baseball is filled with buzzwords that are often used imprecisely. In this series of articles, Mike Richmond explores the baseball’s motion as it travels through the air: How does it behave, and what can a pitcher do to control it? This article looks at the slider to see what causes its very distinctive break.
This is also important for throwing a fastball and other pitches as well.  What do I mean when I say stay behind the ball?   It means don’t be under the ball, too much to the side of it, or overdoing it with your wrist or elbow. Regardless of your arm slot, you will properly get to this point in your delivery by doing everything correctly with your mechanics from the ground up.  From the time your feet first move to deliver the pitch.  
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.
Throw the ball at half of regular speed to your partner when practicing. A really important part of throwing a curveball is ensuring that you get the direction of the spin correct and that you can repeat this at least 80% of the time. As such, throwing at half your regular speed is a good halfway point to get your action started but to minimize the variables you have to battle.[10]
This simple drill keeps the weight back while in the wind-up. Once the pitcher gets used to it, he can develop a nice natural flow, rock, turn, raise, drop, raise and pitch. Then alternate the drill every other pitch. Pitchers who are comfortable with it, can even do it between innings for a pitch or two just to reinforce their proper piece and keep from rushing.
For example, physical forces and properties, such as gravity, friction, velocity, acceleration, and momentum, are constantly at work when a ball is thrown and subsequently hit by a bat. When it comes to a curveball, though, the spin put on the ball by the pitcher brings a couple of other scientific principles into play: Bernoulli's principle and the Magnus Effect.
A slider is meant to be slightly more deceptive than a curveball because it is thrown harder and has spin that more closely resembles a fastball — although it doesn't create as much overall movement. Many power relief pitchers possess only a fastball and a slider in their arsenals — with one pitch setting up the other because of the late deception created by the slider.
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.

5. Release: Releasing a curveball is much different than releasing a fastball. A fastball release is straight out in front of your body. In effect, the way you release the ball is the type of action you want the pitch to have. When releasing a curveball, your wrist will be hooked and your hand will pull down in front of your body. It is important that you release the ball close to your body (Short Arm). The further you release from your body, the less resistance your middle finger will have on the seam and therefore your rotation will be looser. Loose rotation curveballs tend to spin or hang.

Players get on one knee about 45 to 55 feet from each other, kneeling foot on upside-down 10-gallon bucket. The pitcher with the ball will rotate his shoulder toward his throwing partner, bring his arm back with his hand on top of the baseball, use a good circular arm motion, and throw the ball, and popping up and over the bent stride leg, making sure the pitcher bends his elbow and finishes throwing elbow past the opposite knee.

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