The Astros were accused of mounting a camera to look in on the catcher and steal signs real time and then relay the information to the hitter real time.
I am not certain of the accusation against the Red Sox since nothing specific has been recounted. The basic information is one or several players would go to the replay room to see what pitches were being called and to learn what the signs were. Was it when a htter got to second that they could use the information to relay to the batter during an at bat? If so, how much different is that than hitters on second stealing signs while there (which is legal)? I would imagine most teams would study replay footage to learn about signs but also pitcher tendencies. To me to counter the stealing, the teams need to switch signs regularly to defeat this kind of sign stealing. I'm saying, the devil is in the details.