Throughout the course of baseball history, pitchers have experimented with countless different methods for hurling the ball at a batter. During the latter part of the 19th century, the curveball was born in order to deceive batters and give an advantage to the pitcher. The curveball breaks downward as it approaches home plate, and it curves either inward toward or outward away from the batter, thus throwing the batter off balance. Originally considered rule-breaking, the curveball has become a staple in the quivers of pitchers at every level of the game.
Because of the arm motion necessary to effectively throw a curveball, it is not uncommon to see injuries result from throwing the pitch, especially to younger pitchers in their teens or younger. The rotation of a curveball must move forward rather than backward like a fastball, and must also curve slightly sideways. Therefore, the pitcher must allow his arm to move in an unnatural motion that puts stress on the shoulder and elbow. It is generally not recommended that young pitchers throw curveballs until they are in their mid to late teens and their muscles have had sufficient time to develop.
Because of the odd rotation of the curveball, hitters tend to be fooled by the pitch and thrown off balance. A fastball moves toward the batter with strong backspin, allowing a fairly steady trajectory, but the curveball employs the use of an unsteady frontward spin that makes the ball move suddenly downward as it approaches the plate. As the bottom drops out of the pitch, the curveball also swings either hard inward toward a batter, or hard outward away from a batter. Many batters will allow the pitch to go past them without attempting to hit it, thinking it will be called a ball. But at the last second, the ball will break across the plate for a strike.
While a curveball does not move as quickly as a fastball, it can be one of the most daunting pitches to face as a batter. It is not uncommon to see a batter diving out of the way of the pitch, as the illusion fools him into believing the ball is headed right toward him. Only once they have taken themselves away from the prime location for striking the ball do batters realize the ball will break back toward the plate. This results in a strike against the batter.