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.
A curve ball moves in a direction counter to what the batter expects, and will make use of gravity to have it moves farther than it otherwise would (if it was only the spin of the pitch causing the change in direction).
I like the bands at left and right of the screen on that site! All wobbly when you just look at it, yet totally straight when you focus on it.
Would a curve ball "curve" less if it was sewn with white thread instead of red thread, making the rotation more difficult to percieve?