The secret behind the curveball is the grip the pitcher uses when he releases the ball. A proper curveball grip is between the pitcher's thumb and his index and middle fingers. The middle finger must rest on the seam of the ball. To effectively produce the spin of the curveball, snap your wrist when releasing the ball. The spin causes air to flow differently over the ball than underneath.
The top half of the ball is spinning directly through the air, while, the bottom half of the ball travels with the air flow. As a result, the spin of a curveball allows the air traveling underneath to travel faster than the air over top of the ball, creating less pressure. This creates the effect of the ball traveling up or down. This effect is known in physics as the Magnus Effect, which was discovered by physicist Gustav Magnus in 1852. Magnus realized that spinning objects are forced to move sideways.
A curveball appears to move straight towards home plate before suddenly dropping down to the right or left when it reaches the plate. The several inches by which the pitch breaks makes the curveball a much more difficult pitch to hit than a straight fastball.