The pitcher is taking advantage of the Magnus Effect when throwing a fastball. The Magnus Effect is when a spinning sphere effects the air pressure around it. The side of the baseball spinning with the direction it is traveling moves against the air faster, creating more drag and pressure on the ball which causes the air to push on it. On the opposite side of the ball, air pressure is reduced which makes the ball travel easier in that direction when a spinning sphere effects the air pressure around.

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.

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.
Go through your regular wind-up routine, but stop when you get to the kick, holding at your balance point for three to five seconds. Have a friend, coach, or teammate make sure that your leg is waist-height or higher, or have a look in the mirror. If you have trouble holding the stance for five seconds, practice tightening up your abdominal muscles, bending the leg you're standing on and aligning the height of your hips and shoulders.
In the stride phase of the pitching motion, a pitcher should be able to draw an imaginary line from the heel of his back foot, through the ball of his stride foot, and onward to the target. Keeping the lower body aligned in a straight line closes a pitcher's hips, directs the shoulders, and allows the throwing arm to reach the "high cock phase" of its arm path in the back of the pitcher's body. Additionally, if a pitcher lands too far to the glove-side of his body, he will open the shoulder too soon. This causes the pitch to be low and outside while creating stress on the arm and reducing velocity. If a pitcher lands too far to the throwing-side, he will inevitably have to throw across his body making the outside part of the strike zone difficult to hit. Plus, if a pitcher throws across his body, he creates an increased amount of stress on the arm.
While this partner is rotating their arm in and out the other athlete will hold the KB Powerbands with both hands extended out in front of their body. With knees slightly bent, back straight, and shoulder blades pinched together this athlete will squeeze their abs and hold this position. As their partner rotates the athlete will feel tension build in the abs. Athletes must keep abs tight to avoid being pulled or rotating the hips or arms.
Practice pitching standing up once you get more comfortable with the throw. After practicing on your knees and getting the hang of throwing a curveball, it's a good idea to try pitching while standing up so you can practice throwing with a fuller motion. This is a more realistic representation of how you will actually be pitching when you play.[11]
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.

Simply place your fingers the same way you would a curve using your index and middle fingers putting them close together. Thumb and middle finger split the baseball in half as you should do with most pitches.. Keep your ring finger and your pinkie off to the side as they shouldn’’t make any real contact with the ball. Place a little more pressure with your thumb and middle finger as your index finger puts little to no pressure on the baseball.
a slider spins more toward the vertical axis and hence tend to move across the horizontal plane with a slight to moderate downward angle (otherwise it’s a fat pitch, not a slider) L to R from the vantage of the mound for a LHP and R to L for RHP. A great R hand slider will look like a fastball inside to a RH batter and wind up moving away and down from him so the catcher catches it on the low outside corner of the plate and the batter is completely fooled and half way back to the dugout by then after whiffing at it. A curve spins toward the horizontal axis so that the rotation produces downward force (bernoulli) and hence it “breaks” or curves more than “slides”. A great curve will break 12 to 6 or down the face of a clock. A great slider is more 2 to 7 for comparison. Both wickedly effective pitches when thrown in the right count and settings.
Once you've perfected your fastball, learn how to throw a curveball to improve your pitching skills. A well-thrown curveball looks like a fastball, but it spins in the opposite direction, causing it to "break" in a different direction before reaching the hitter. With a little luck, the hitter will swing early and miss the ball. To master this skill, you'll have to perfect your basic curveball, the straight curveball, and the knuckle curveball.

“Dick’s Scientific Formula For Big League Pitching Mechanics Package has given me the knowledge I need as a pitching coach to help young people succeed. I highly recommend it to any pitcher Little League through college. From mechanics to conditioning to the mental aspect, everything he does is top notch. His program helped our pitchers go 29-1, have a 0.80 ERA last season, and win a State Championship.”
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.

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 😉