The forkball is a pitch thrown when the ball is wedged between the pointer and middle fingers. Much like a split-finger fastball, the pitch will drop. The difference being, a forkball is jammed deeper into the hand and is a slower-thrown pitch, enabling the ball to tumble as it approaches the batter.
Jack Morris in the mid-1980s and early 1990s had a very effective forkball. More recently, Tim Lincecum of the San Francisco Giants has dominated by using an effective forkball.
By utilizing a forkball in conjunction with a two seam fastball and split-finger fastball, pitchers will do a good job of throwing off the batter's eye level and timing and improve their performance by implementing the forkball pitch as a variety in their repertoire.
This is how you throw a forkball.