The slider is an effecitive pitch in three ways. First, it is going at about the same speed as the fastball...a bit slower but not a lot. Secondly, there is a slight break to the pitch of about 4 to 8 inches. Third, it is not a difficult pitch to throw.
Get a Grip
The grip of the ball is almost identical to your fastball. Notice that my middle finger is on a seam as well as my thumb. The major difference in throwing a slider is what you do right before you release. I tell my kids that you must think you are throwing a fastball up until this point. Let me explain...
An instant before you release the baseball you will snap your wrist on the side and your pointer and middle fingers will come over the top of the ball, This will give it just enough necessary spin to create the small break yet keep the ball speed up. This sequence is shown in the last four figures to the right.
The Physics of the Slider
In talking about when to throw the slider, we need to understand what it does. As discusses above we now know that. So a pitcher will want to throw a slider after a good fastball inside. He will throw a slider away. This is done because the batter has just seen something straight and hard, and now he is going to see something hard and breaking a bit. The slider can also be used as a set up pitch on top of what we just described...an out pitch.
We can think in the opposite way: a pitcher throws a hard slider away off the plate that the batter swings at it and misses. Well what can he come with next--a hard fastball in on the hands or on the corner. That pitch will freeze up the batter or at best he may fist it into the infield if his hands are quick enough.
It's All About the Location
The slider then can be a deadly pitch if used correctly. It is however very easy to hit if you leave it out over the plate. That is because it is about the same speed as a fastball therefore the hitter does not have to worry about dumping his wieght on it like a curve or other offspeed pitch. You don¹t want to throw sliders to hitters who go the other way. That is because they will slap singles to the oppositve field consistently. Most of those hitters are said to be contact hitters.
Now this is all good for the same side hitter as pitcher...ie LH hitter and LH pitcher. So what good does the slider do if I am a LH pitcher and the hitter is RH? In this case the slider slides into the batter and not away from him. Good question...A slider in on the hands can be very effective because the batter is usually not prepared for the little extra break at the end. Therefore his swing is not going to be adjusted to that...more often then not the batter will be jammed.
This pitch is a lot like the cut fastball which I have discusses also. Take a look at that and see which best fits your needs.