The slider is a very effective pitch at deceiving the batter.
The slider is a combination of a fast ball and a curve ball that slides more horizontally rather than vertically. Some players that have thrown effective sliders in recent years are C.C. Sabathia and previous to that David Cone, who in 1999 threw a perfect game, using a slider grip. Bob Gibson, historically, was an outstanding pitcher who used the slider.
The slider is thrown effectively by putting the middle and pointer finger against the seam on the baseball. And it is thrown with a quarter turn of the wrist, pronating the thumb upwards upon release. It must be thrown with a fast ball arm speed so as the batter will not be tipped off that an off speed pitch is coming. A slider is usually three to five miles an hour slower than a fastball and has late break as Ron Guidry in the late 1970s showed when he struck out 18 batters of the California Angels with a nasty left-handed slider.
These are some tips for throwing an effective slider.