With my fastball, I'm trying to keep my two fingers behind the ball as long as I can to pull down on it and create as much backspin as possible. With the curve, instead of trying to stay behind, it's almost the opposite. At the very end of the release, you try to get your hand in front of the ball to create that topspin, which makes it break. You're rolling your hand forward and down off the side of the ball as you snap your wrist.
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.

When thrown, try to manipulate the pitch to come off of the thumb-side of your index finger – NOT your index- and middle-fingers, as with a two-seam fastball – because a two-finger release will cause the pitch to balance out, which reduces the spin that you are looking for. Most good slider pitchers grip the outer-third of the baseball and cock their wrist slightly, but not stiffly, to their throwing hand's thumb-side upon release of the pitch. This enables a pitcher to apply pressure to the outer-half of the ball with the index finger. Avoid any twisting of the wrist upon release.
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.

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.
A pitcher's should stride at a minimum 80% his height towards home plate during his fastball delivery. On the curveball and change-up, his stride should be six to eight inches less than his height. For example, if a pitcher is 5 feet, 10 inches tall, then his stride toward home plate on the release of the baseball should be 5 feet, 2 inches (or thereabouts).
Great baseball pitching is your team's first defense. So it's important your pitcher be their best. But while you may think good pitching involves natural talent, think again! There's no question that a laser lob calls for lots of balance, strength, agility, speed, and accuracy. With the right drills, any pitcher can accomplish that. Effective pitching is a learned art. Understanding the mechanics of pitching from the ground up, and choosing drills that fine-tune both the lower and upper parts of the body are sure to take your pitcher from average to ace!
The spinning action created when the pitcher releases the ball is the secret behind the curveball. This spinning causes air to flow differently over the top of the ball than it does under the ball. The top of the ball is spinning directly into air and the bottom of the ball is spinning with the air flow. The air under the ball is flowing faster than air on top of the ball creating less pressure, which forces the ball to move down or curve. This imbalance of force is called the Magnus Effect, named for physicist Gustav Magnus, who discovered in 1852 that a spinning object traveling through liquid is forced to move sideways.
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.

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.

Step 2: This grip provides maximum rotation movement, more than any breaking pitch. However, many pitchers who are learning this pitch for the first time aren’t comfortable with the ‘tuck’.  It’s not natural at first to tuck your index finger into the baseball. I recommend, preferably during the off-season, to practice tucking your index finger into the baseball. Do it while you’re watching TV!
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.