The innovator of the slider is debated, but some credit Chief Bender as the first to use the slider, also George Blaeholder was credited with using it with the St. Louis Browns then called a "nickel curve", in the 1920s.[6] Others have also credited George Uhle with developing the pitch.[7] Bender used his slider to help him achieve a no-hitter and win 212 games in his career.[8] Bender was the first pitcher to win six World Series games.[6]
Place your thumb. Place your thumb under the opposite inside seam of the ball. The further your thumb is from your other 2 fingers, the more the pitch will drop. The closer your thumb is to your other two fingers, the more it will slide. If your index and middle fingers are at a 10 or 11 o'clock position, your thumb should be at a 4 or 5 o'clock position.

I didn’t add this into one of the original 13 best baseball drills only because I already mentioned it above.  This is in the first video where I share my two favorite baseball pitching drills.  But I figured I post this video too because I explain it in a little different of a way.  So, if it helps someone understand a little better, then good.  That’s what I’m here for 😉
Michael grew up on the South Shore of Massachusetts, but rebelled against his parents by rooting for the Orioles (eventually, he came to his senses). After receiving his Ph.D. in Astronomy from UC Berkeley, he spent five years as a post-doc at Princeton working on the Sloan Digital Sky Survey. He now lives in Rochester, NY, studying supernovae and listening to baseball games far too often.
The downside risk makes some experts advise pitchers to avoid drills altogether. But I've seen them produce tremendous results in too many of my pitchers to discredit them. What athletes, parents and their coaches need is a way to navigate the landscape of pitching drills. The following will help you know what to look for so you can separate the good from the bad.
For these explosive youth pitching drills athletes will need a set of Kbands attached just above their knees. Athletes will move through two phases of this youth pitching drill, tuck jumps and split jumps. This unique combination of exercises allows athletes to focus on building baseball power in both legs while also isolating the legs and forcing them to individually develop these power characteristics.

It is important when throwing a slider, or any breaking pitch in baseball, not to come "around" the baseball. When the pitcher "comes around" the ball, the pitcher puts extra tension on his pitching arm to throw that pitch. A slider is thrown with a regular arm motion, just like a fastball. Slider movement is a direct result of the fingertip pressure and grip. The pitcher may visualize throwing his fingers at the catcher in order to improve follow through and finish the pitching motion.
Do you know a baseball pitcher or someone who coaches baseball? Given the popularity of baseball, there's probably a good chance you know someone who can help you learn to throw a great curveball. Find a pitcher or a coach and ask them to meet you at a local field to give you a few pointers for throwing a great curveball. Enjoy giving your throwing arm a good workout!

The arm action on this pitch is a little abbreviated at the end. Instead of getting a nice long arc of deceleration and finishing throwing elbow outside of your opposite knee (as with your fastball), you'll want to bring your throwing-hand elbow to the opposite hip. This, of course, shortens your follow through, but allows you to really snap off the pitch.

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.

Mostly the speed and amount of break. A curveball is thrown more slowly than a slider, and the amount of curve on its trajectory is more even. From the batter’s perspective it looks like the pitch is falling off a table. On the other hand, a slider is thrown almost as hard as a fastball. Its trajectory is much sharper than a curveball’s, its break becomes noticeable about 2/3 of the way towards the plate. To a batter it looks like a fastball, then suddenly breaks sharply.
The difficulty with this pitch isn't from the pitch itself. In fact, most pitchers feel this grip gives them the most rotation – and most movement – of any breaking pitch. However, many pitchers who are learning this pitch for the first time, aren't comfortable with the "tucking" part. It's not super comfortable at first to tuck your index finger into the baseball.
One common situation arises when a pitcher faces a “same-handed batter” – a righty pitcher matched up against a right-handed batter, or a southpaw against a left-handed batter. In these matchups, a fastball breaks toward the batter, while sliders and curveballs break away from the batter. If the pitcher aims his slider toward the outer half of the plate, it tends to move farther and farther away from the batter as it approaches him – perhaps ending up far outside the strike zone.
In the final phase of this preventative pitching drill the athletes will both face forward. The athlete performing the shoulder portion of the pitching drill will place the KB Powerbands in their outside hand as they laterally lunge and extend their arm away from their body. During this motion it is important athletes use a controlled motion while keeping the arm extended and moving in a straight line. Athletes need to maintain greater control and a slower pace as they bring the hand back toward the body in a controlled motion.
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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Later that summer, the kid would commit to Vanderbilt University, better known as “Pitching U’ because of the plethora of first-round draft picks they were pumping out under the tutelage of pitching coach, Derek Johnson. (Derek Johnson is a member of the BaseballThinkTank Advisory Board and author of the best selling book, “The Complete Guide To Pitching.”)
B) Twist or snap your wrist: In this brief pitching video I explain this myth in greater detail. If you twist your wrist right before release of the baseball you will experience elbow problems in the long run. In fact, you will know that it is wrong to throw the ball like this to begin with because your arm will tell you it’s wrong. Why do pitcher’s continue to throw like this? Again, this method works for some pitcher’s because it does impart rotation on the ball. The wrong rotation, but pitchers find success with it so they continue to use it.

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.
A right-handed pitcher snaps his wrists in clockwise rotation when delivering a curveball (he grips the ball with his middle finger near the top of the ball and his thumb on the bottom---but releases the pitch with his thumb near the top).  A pitcher throws a slider with smaller wrist-break.  He imparts spin by gripping the ball "off-center" (that is, he grips the ball with his middle finger at "2 o'clock"-- instead of 12 o'clock)
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]
I learned this pitching drill from Coach Scott Birchler who was a high school coach at our rival high school but as I got older and made the transition from player to coach, he helped me a lot. This pitching drill that he taught me is great for any pitcher who wants to be more consistent with their curveball or slider and fixes two of the biggest problems in guys who throw these pitches, dropping that elbow and not finishing the pitch.