During a game, the catcher and the pitcher have to both know what pitch is being thrown so the catcher can catch it - it’s hard to catch breaking balls unless y

During a game, the catcher and the pitcher have to both know what pitch is being thrown so the catcher can catch it - it’s hard to catch breaking balls unless you know to expect one. So the game has evolved that the catcher normally “calls” games, eg decides what pitch the pitcher is going to throw. Catchers make careers on how well they call games. They are the smartest players on the field, and have to remember the stats and approaches for every batter on the other team, not only for the starting pitcher that day but also all the relievers that might be used in a game. They communicate the pitch to be thrown to the pitcher using hand signals, normally with the non-catching hand on the inside of their thigh so the batter can’t see the sign. But anyone behind the pitcher can of course see the sign, and after seeing a few tens of pitches at most can figure out which sign corresponds to which pitch. Knowing what pitch is coming is a significant advantage to the hitter. It’s legal for a runner on second to watch the catcher and convey those signs to the pitcher, but in MLB it’s strictly illegal to use “technology” - meaning, mostly, optics or electronics.