For those who can't read the article, here are some interesting quotes:
Three people who were with the Red Sox during their 108-win 2018 season told The Athletic that during that regular season, at least some players visited the video replay room during games to learn the sign sequence opponents were using. The replay room is just steps from the home dugout at Fenway Park, through the same doors that lead to the batting cage. Every team's replay staff travels to road games, making the system viable in other parks as well.
And:
"It was like having an open-book test and the open book is right there next to you and the teacher says, 'Don't look at the book,'" said one former player. "Whatever is available to teams, they're going to take advantage of it. Major League Baseball knows that. If you have this technology that's available where you have 20 cameras on the field, cameras that can look at signs, I mean, it doesn't take a rocket scientist to see: Oh, if I'm in the video room and I see the guy's signs, you're basically playing the same game now that was played when I first came into the league and there was a guy on second base. You're trying to break the code."
And:
A staff member in the Red Sox's video replay room would tell a player the current sign sequence. The player would return to the dugout, delivering the message on foot, rather than through a wearable device or a phone.
"There was constant movement," said one person who was with the 2018 Red Sox. "They were always trying to figure out the system."
Someone in the dugout would relay the information to the baserunner, leaving the runner with two easy steps: Watch the catcher's signs and, with body movements, tell the hitter what's coming.
In daily hitters' meetings, Red Sox players and personnel would review their communication methods for that day.
The runner would let the hitter know if he was aware of the sequence. "Put two feet on the bag or look out into center field, and do something that's subtle," as one Red Sox source described it.
The runner stepping off the bag with the right foot first could mean fastball; left foot first, a breaking ball or off-speed pitch.
Such a system was far more difficult for opponents to detect than banging on a trash can. It also had a semblance of propriety, incorporating old-school, legal practices: A runner on base still had to use his own eyes before he could put the contraband information to good use.