The difference between a slider and curveball is that the curveball delivery includes a downward yank on the ball as it is released in addition to the lateral spin applied by the slider grip. The slider is released off the index finger, while the curveball is released off the middle finger. If the pitcher is snapping his wrist as he throws, and the movement is more downward than sideways, then he is probably throwing a curveball or slurve, and not a true "slider".
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.
The drill is used to develop great pitcher control by having the pitcher throw strikes at varying distances. The drill has a catcher set at a stationary plate. The plate never moves. The pitcher should begin throwing at a distance 1/2 of his normal pitching distance. You should have 6-8 distance markers with the first being at his starting point and the longest being twice his normal pitching distance. The markers should be at 10 foot intervals and in a straight line with the plate. The object of the drill is to develop control by gradually moving toward and away from the targeted strike zone. The pitcher is required to throw 1-3 strikes from each marker before moving to the next. The catcher serves as the umpire. Variations of this drill may be to have 1-3 pitchers working and competing against each other. The drill teaches them to work fast, concentrate, and execute a perfect pitch. Make sure your pitchers are in condition for this drill. They will find that throwing strikes from longer distance requires great mechanics and builds arm strength. Make sure your players stretch and warm-up first.
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.

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.
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.
Asking if a kid can see that slider spin is occurring is a common question I will bring up in lessons. Having a thought process and feeling with what they are doing is something that he or she can control and fix from throw to throw but it is not often taught. We like to have kids visualize and react to what is being said and into what is being felt. When trying to recognize the slider spin, the catcher should be able to see where the thrower is missing with his fastball. Seeing the ball out of the thrower’s hand and knowing that a slider has clockwise spin on the baseball you will be able to clearly tell that they released the ball wrong.

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)


Let's begin. A pitcher will stand perpendicular to a straight line (like a foul line in the outfield grass or line on a gym floor). If the pitcher is on the pitching mound itself, he can use his spikes to drag out a straight line in the dirt 8-feet long and perpendicular to the rubber (i.e. directly in line with home plate). Then, he simply marks out the distance of his height and drags out a second line in the dirt--only this one is parallel to the pitcher's rubber. If the pitcher is not on a mound, he will simply place a second object like his hat on the ground. This will mark the distance he should be striding toward his target.
In order to throw a proper Curveball, it is necessary to get "on top" of the ball and then spin it downwards. The grip is pretty simple: just put your index and middle fingers together and then place the middle finger alongside the Blitzball seam. You should get a good, tight grip on the seam with your middle finger so that you can really get some leverage on it.

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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