THE RED DOT. For many years, I have heard about the red dot on the baseball that a batter is supposed to see when a slider comes out of a pitcher’s hand. There was even a phenom whose success was attributed to the fact that there was no red dot when the pitcher threw his slider. Having good enough eyesight to pick up the spin on a pitch when it leaves the batter’s hand is a huge advantage for a hitter. (It means little to fans today, but Ted Williams was supposed to be able to read the fine print on 78 RPM record.) The earlier that you can see the way that the ball will break, the easier it is to square it up.
Here is an article by Max Axisa on CBS sports.com which includes graphics of what the batter sees when a pitch leaves the pitcher’s hand. You can also see what a curve ball looks like.