A curveball will slowly shift by a couple of feet at most, but to the batter it seems to suddenly jump several feet.  Watch this animation of the Best Visual Illusion of the Year winner to experience how this occurs.

In baseball, a curveball creates a physical effect and a perceptual puzzle. The physical effect (the curve) arises because the ball’s rotation leads to a deflection in the ball’s path. The perceptual puzzle arises because the deflection is actually gradual but is often perceived as an abrupt change in direction (the break). Our illusions suggest that the perceived “break” may be caused by the transition from the central visual system to the peripheral visual system. Like a curveball, the spinning disks in the illusions appear to abruptly change direction when an observer switches from foveal to peripheral viewing.

Link - via josephfosco

From the Upcoming ueue, submitted by josephfosco.