Apologies in advance for the absolutely terrible/fantastic title.
The winner of the 2009 contest for the Best Visual Illusion of 2009 is particularly appropriate tonight as the Yankees strive to hand the Phillies their second championship in as many years. The prize was won by a team that has identified the core difficulty in hitting a curveball: it's not where you think it is.
The illusion arises because the seams on the ball spin sideways when a curveball is thrown. When the ball is seen in the batter's peripheral vision, the brain infers from the sideways motion of the seams that the ball itself is traveling sideways, even if it is not. Conversely, when the ball is seen by the batter's central vision system, the sideways visual cue is ignored, presumably because the image of the ball itself on the retina is more a more convincing indicator of it's position.
Critically, not only does the ball appear to drift when viewed by the peripheral system, but when it transitions from central to peripheral vision it appears to jump from one position to another! So the combined effect is: the ball is physically curving through space AND when it reaches the peripheral vision system it is perceived to jump from one position to another AND while in the peripheral system it appears to drift sideways in addition to its actual curve.
The authors argue that this explains the phenomenon of curveballs "breaking" suddenly. A curveball which hangs - that is, one that doesn't break - likely isn't spinning at the right speed to set off the illusion.
But don't take my word for it: try the illusion out for yourself.