Gripping a curveball is simple. Place your thumb on the bottom seam of the baseball and then place your middle finger directly above your thumb; splitting the baseball in half with thumb and middle finger. Your index finger is placed right next to your middle finger. Make sure your index finger applies no pressure on the ball. When you start throwing the curveball you can experiment how tight you want to grip the ball. If your grip is too tight the ball can “squirt” on you or it will not make it across the plate. If your grip it too loose you will lose complete control and the ball won’t even know where it will go. Thumb and middle fingers are the only two fingers that apply pressure on the baseball.
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".

A 4 seam fastball is the most common pitch and the ideal grip for a position player as well on the transition from glove to hand. WHY? Because at release point the finger causes backspin on the baseball. The result is the ball does not drop as much as otherwise, without backspin. In other words, a 4 seam fastball is really appearing to defy gravity and travel more in a straight line. A 2 seam fastball is thrown with similar backspin but again on 11 o’clock axis.

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.
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.
A successful major league batter gets a hit only 30 percent of the time he comes to bat. One of the ways pitchers lower these chances even further is by throwing a curveball. A curveball is a pitch that appears to be moving straight toward home plate but that is actually moving down and to the right or left by several inches. Obviously, a pitch that curves is going to be harder to hit than a fastball that is moving straight.

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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 😉