YOU HAVE TO DISRUPT THE SLIPSTREAM.

The two-seam fastball does so more effectively than the four-seam pitch.  "Because of this, the maximum amount of drag can be achieved, and after a certain number of rotations, enough negative Magnus effect force will have been built up to cause a sudden downward pull."  Not that my eyes, let alone my reflexes, are good enough to pick up the visual signature of the pitch.  "The four-seamer on the other hand is a repeated interval of seam and no-seam ball surface, which isn’t enough to generate a negative Magnus force."

In baseball, as in economics, it is often what goes on at the margins that is most important.