When I was growing up and had dreams of being a big league pitcher, I learned through one source or another that Dave Stewart had a mean streak as long as his arm and a nasty forkball. Forkball? I thought. What's a forkball?
A forkball (similar to the split finger fastball) is thrown by placing the ball as deep in the space created by the spread index and middle fingers as possible, in essence "forking" the ball between the fingers. The tighter you get it, the harder you'll have to throw to get it out of there, which in turn creates both deception through velocity (something akin to a changeup) and through motion (since the ball will sink more.) When releasing the ball, some players choose to snap their wrist in a straight 12-to-6 motion, while others don't snap it at all, instead using the wrist time to exert extra momentum on the ball, giving it a harder break near the end.
(It is important to remind younger players of the dangers of throwing motion pitches on undeveloped elbows and wrists. The forkball is no exception and, besides, it's virtually impossible for anyone under 15 to get the ball far enough into the index-middle wedge to throw a decent forkball. Most major leaguers can't throw it, either.)
A forkball is perhaps the most dangerous fourth pitch in baseball. There is, basically, no such thing as a bad forkball. Unlike the changeup, which can hang in the air forever, or the knuckleball that doesn't catch a good draft, or the slider that never finds its way back into the strike zone, the forkball is a very accurate pitch that emphasizes location and control over power and trickery. Some of the best forkballers - Hideo Nomo, Kevin Brown, and Stewart, to name a few - are also masterminds of the pitch count and the first pitch strike theory of baseball. In short, the forkball is a hard pitch to master without first learning the basics of ball placement and good control, but is an invaluable out pitch when used correctly.
For a picture showing the forkball grip, as well as the grips to a majority of the other common pitches in baseball, see http://www.jsonline.com/sports/brew/sat/rep22198.stm.