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 throw a slider, start by gripping a U-shaped seam on the ball with your index and middle fingers. Put your thumb under the opposite inside seam, but remember to conceal your grip in your glove so the batter can't see what you're throwing. To throw, cock your wrist towards the thumb side of your throwing hand, then pivot from your back foot towards home plate. As you throw the ball, apply pressure with your index finger to ensure a late break and snap your wrist up to down to make the ball drop over the plate.

Being able to identify if your son or daughter is trying to throw a 4 or 2 seam fastball and throwing a slider instead is also key. They are different spin axises that affect a consistent ball path. So when playing catch learn how to pay attention to spin on the baseball along with consistent movements. Ask your son or daughter what they are feeling on a throw to throw basis. What finger did the ball feel like it came off? Did you see the spin on the ball? Where was your eyesight? Your arm swing looked a little stiff, did you feel that?
Master the "two-seam" fastball. Once you effectively use the two-seam grip and arm motion, creating sinking action on the ball, you will be better prepared to throw sliders. For the two-seam fastball, place your index finger and your middle finger directly on the narrow seams of the ball -- the top of the "U" in the stitching. Put your thumb on the bottom of the ball, on the smooth surface of the ball directly under the fingers. Grip the ball tightly to create friction and movement. Use the same arm motion as on a regular or "four-seam" fastball.
Choose a grip (start with the standard finger on top of the ball variation) and see how it feels, and get some feedback from a qualified partner on how it looks. Then, tinker. Try others, and see which grip works best for you. Remember: NO grip is best, and any can produce an amazing curveball – it just depends on the person and his level of comfort with it, and his unique way of throwing.
A well-timed curveball can be highly beneficial to pitchers, but a curveball is pretty useless if the batter knows it's coming so that he or she has time to adjust to the swing. For that reason, it's important that pitchers not only master the grip and motion of the curveball but also the secrecy of the grip itself, which is necessary for fooling the batter.
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).
At the end of this corresponding video, you will look at a short clip from the Best of the Best.  Where a Slider thrown by Clayton Kershaw, is featured from a breakdown video/post I recently did of the lefty.  In slow motion, you can see that he has done everything correctly in his mechanics to enable him to Stay Behind the Baseball, Release it ‘Out in Front, and Throw it Downhill.  As for result, well it speaks for itself.
Place your index and middle fingers. Grip the baseball with your index and middle fingers placed tightly together across an outer seam of the ball located at the horseshoe or U-shape seam. For right-handers, place your middle finger across the right half of the seam. Left-handers should do the opposite: place your middle finger across the left half of the seam. This should position your fingers towards the outside of the ball (off-center).
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.

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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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