What is the method of ``sign stealing using Apple Watch'' that was once rampant in the majors?

In Major League Baseball (MLB), the world's top professional baseball league, `` sign stealing '' using Apple Watch became a major problem in 2017. Here's an easy-to-understand diagram summarizing how some teams were stealing signs using Apple Watch.

Apple Watch and Cheating in Baseball - The Watch Doctor
https://blog.watchdoctor.biz/2018/10/29/apple-watch-and-cheating-in-baseball/

'Sign stealing' in baseball is the act of intercepting signs shown by a catcher to the pitcher or signals shown by a coach to runners on base and transmitting the information to the players. For example, by stealing the catcher's signals, it is possible to know in advance what kind of pitch the pitcher will throw next, which is a big advantage for the batter. Signs are generally posted in visible locations, so they may be visible even if you don't know what they are saying. It is expressly prohibited.

In fact, sign stealing using Apple Watch became a problem in 2017, and the New York Yankees submitted video evidence to the commissioner after the Boston Red Sox stole signs using Apple Watch. The Yankees point out that ``The Red Sox stole signs by having their staff watch the game on TV and sending information about the signs they obtained as a message to the coach on the bench via Apple Watch.''

'Sign stealing' using Apple Watch discovered in Major League Baseball - GIGAZINE

MLB's Commissioner's Office has determined that the Red Sox used an Apple Watch to steal signs and has fined the team. After that, the coach of the Arizona Diamondbacks also wore an Apple Watch on the bench, so he was interviewed by the MLB Commissioner's Office.

The figure below summarizes how sign stealing was actually carried out using Apple Watch.

When there is a runner on second base, the catcher disguises the contents of the sign because there is a risk that the runner on second base will steal the sign.

During games, the Red Sox check replay footage from a center camera placed on the field, match the catcher's signatures with the type of pitch thrown by the pitcher, and analyze the signatures when there is a runner on second base. .

An outside video analysis team sends the results of the analysis of the signs to the coaches on the bench. Coaches will receive the results of the signature analysis on their Apple Watch.

The analysis result of the sign is transmitted from the bench coach to the second base runner with a hand sign...

Furthermore, by having the runner on second base give signals to the batter, the pitcher will be able to understand in advance what type of pitch he is going to throw next. It is clear that for MLB's top batters, knowing the type of pitch they will throw next is a huge advantage.