from mlb writer Ken Rosenthal, in The Athletic: ”They used a different system on the road, decoding signs during games from feeds to the video replay room, then relaying the sequences from the replay room to the dugout to a runner at second base to the hitter.”
Further he notes, “the use of that algorithm would have remained legal if it had not been deployed live during games”
Here’s another explanation from a different source, Jared Diamond in the WSJ story: “Codebreaker would operate by having someone watch a live feed of games and log catchers' signs into the spreadsheet with the pitch that was thrown. The algorithm would break down the correlation between signs and pitches. That later evolved into employees banging on trash cans just behind the dugout to notify batters which pitch was coming. Diamond noted the use of Codebreaker extended to road games as well as home contests.”
“ According to the Wall Street Journal, "Codebreaker" was used during both home and away games into the 2018 season.”