How to throw a slider like Zack Grienke - Grip Tips
What's so great about the slider?
The slider is one of the most popular pitches in baseball because it is both fast and has movement. A good slider is only 5 to 10 mph slower than a fastball from the same pitcher, but it breaks down and side to side. It is most effective when thrown to a same-handed hitter. For instance, left handed pitchers want to mix in a lot of sliders to left handed hitters, because the ball will break away from the hitter.
Some of the great slider pitchers include Bob Gibson, Steve Carlton, David Cone, John Smoltz, Randy Johnson (his slider was called Mr. Snappy) and Rollie Fingers.
Now I'm going to show you how to grip and throw a slider.
The slider grip
The slider grip is similar to the curveball grip. You should place your middle finger along a seam with your index finger pushed up against it. Your thumb should be on the other side of the ball and either on a seam or just to the right of a seam.
Throwing the slider
As you bring your arm forward and just before you release the ball, you're going to snap your wrist clockwise (for the right handed pitcher) and hard downward. Imagine you are grabbing a chain hanging in front of you and tugging it down. This is different from a curveball motion because you are not trying to bring your fingers over the top of the ball. You want the ball to spin sideways and slightly forward.
I've found some videos of good sliders to show you as examples. Examine them carefully.