Curveball
The curveball is thrown with a grip and hand motion that induces extra rotation on the ball causing it to "break," and fly in a more exaggerated curve than would be expected. The pitch is slower than a fastball, and this difference in velocity also tends to disrupt the hitter's timing.
Good curveballs often seem to drop sharply with a sharp rotation as they reach the plate, making the batter swing above it. The rotation on a curveball should be in the opposite direction as that of a fastball so the hitter cannot easily distinguish between the pitches; as such, the drop on such a ball should be 12 to 6 on a watch (although many pitchers are successful with a curve ball that breaks down and away from the batter, rather than straight down).
Ideally, a curveball will have the most break when it reaches the plate, thus making it hardest to hit. A curveball that fails to break soon enough is called a "hanging curve" and is much easier to hit. Also, batters anticipating a curveball may try to move forward (closer to the pitcher) in the batter's box to hit the curve before it breaks.
The curveball rotation is produced when the pitcher snaps his wrist downward at the moment of release, causing the ball to "roll" off the pitcher's hand. The palm of the pitcher's hand typically faces up during the follow through after releasing a curveball.
A right-handed pitcher throwing a curveball is toughest on a right-handed batter, as the pitch will ideally break down and away from the hitter, as opposed to inside on his hands (that's what would happen if the right-handed pitcher threw a curve to a left-handed batter). Most batters prefer pitches down and in, instead of down and away, which allows managers to exploit matchups late in games. If a team has three right-handed hitters coming up in the 8th, the opposing manager will usually go with a right-handed set up man because his curve is going to frustrate those hitters more.
For some reason, left-handed pitchers are more naturally inclined to throw slower, wider breaking curveballs than righthanders. This is apparent by the number of left-handed pitchers who throw high-looping, lower-velocity curveballs. It is speculated that left-handers have better natural pitching mechanics that are more conducive to being able to make the curveball spin more when thrown.