Does your son or daughter have slider spin when throwing a fastball? A common problem in youth baseball is not informing kids that slider spin on a fastball is not ideal for throw/pitch efficiency.  How can you identify if your son or daughter has slider spin on the ball in their throws? First thing is knowing what you are looking for when you’re playing catch in the backyard. If your son or daughter is a righthanded thrower, the first good indicator of slider spin is YOU may be catching the ball on the right side of your body consistently pulling the throws across your body.  A slider has a clockwise rotation and spins on an axis that faces to player indicated by the blue line in the gif below. When playing catch you will be able to identify slider spin by seeing a circle in the middle of the ball. For righty or lefty throwers a 4 seam fastball should have close to perfect vertical spin/12 o’clock to 6 o’clock spin. A  2-seam fastball’s axis, in comparison to a righty, points towards 11 o’clock.  For young kids slider spin when throwing a 4 seam fastball is not horrible for them but can become a bad habit to break. Plus we want to have the correct spin on the ball, especially as a pitcher. In this article, we will identify slider spin, what causes it, and how we can help eliminate it from 4 seams and 2 seam fastballs.
When you throw the Curveball, you want to cock your wrist at a slight angle while keeping your middle finger high up on top of the ball so that you can roll your wrist downwards when you release the ball (almost like pulling down the string on a window shade). As a result, the Curveball should spin forward as it approaches the batter, giving the Blitzball a great downward movement. In terms of delivery, some people like to come almost straight over the top to get that great "12-to-6" / Barry-Zito-style drop (drops almost straight down) on their curveball, but this is difficult to do. Most people get better results with something closer to a three-quarters delivery for a "1-to-7" break (a pitch that drops and curves to the side). Try experimenting with different release points and arm slots to find the pitch that works most effectively for you.
Don't worry about wrist motion; keep your wrist firm and worry about the angle of your arm. This will maximize bottom-left break if you are a right-handed pitcher. The side of your hand should always face the batter, and your wrist should always finish at your left hip (again, right-handed). Also, try to aim at the left shoulder of a right handed batter, as this is a good aiming spot for the break to be efficient. For more information, you can refer to this article: How to Throw a Curveball.

Pitchers and catchers stand at regulation distance from each other. The catcher stands behind home plate, and a batter stands in the batter's box. The batter should alternate between a left-and right-handed batting stance after five pitches, but not take any swings. The catcher calls balls and strikes. Have the batter wear a helmet in this practice session.
Certain relief pitchers specialize in this type of slider. Known as “Lefty One Out GuYs” (LOOGYs) or “Righty One Out GuYs” (ROOGYs), they are brought into a game to face a dangerous same-handed batter. They usually feed the opponent a steady diet of these “runaway” sliders, tempting the batter to swing at a ball that looks like a strike – but isn’t.
Next, place your middle finger along the bottom seam of the baseball and place your thumb on the back seam (as shown in the middle picture above). When this pitch is thrown, your thumb should rotate upward, and your middle finger should snap downward while your index finger points in the direction of your target. This, of course, is the reason this pitch is great for beginners: the ball goes where your index finger points. The beginners curveball helps to align your hand and ball to the target.
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.
Your wrist and forearm position with the slider at release is only slightly different than the curve. With your curveball grip your palm is facing you as you bring the ball out of your glove.: Throwing a slider is similar other than your palm facing you, it is slightly turned in. Using your fastball pitching mechanics with identical arm speed and arm slot, release the baseball in front of you.
The arm swing and finish is the hardest thing to correct in a thrower besides having a feel for which finger the ball is coming off of through the throw. Lucky Baseball Rebellion has developed some fairly simple concepts to allow your child to efficiently enhance upper body mechanics and arm swing.  Here is a #TransformationTuesday tweet from Baseball Rebellion showing how a forty minute lesson can help your son or daughter with arm swing mechanics.
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.

To throw start with your dominant hand facing away from the catcher facing sideways.  Bring up your front leg.  When you do this at the same time bring up your throwing arm with the ball facing away from the catcher 90 degrees to your shoulder like an L shape.  Then bring your arm down to your knee and release at the time necessary.  It should look something like the video.  He doesn't bring his arm and leg up at the same time but everyone's throw is a little different.

Start with the hands together, ball in the glove. The legs are positioned in the exact same manner as the high-cock drill, toes of the lead leg facing the target. The legs remain in this permanent, shoulder-width-apart position throughout the drill. However, the back foot will pivot onto its toes when the ball is released (like when you pivot your back foot during a golf swing or baseball bat swing). However, the distance of the two feet remain the same.