The cheating aspect is using any outside technology. A runner on second isn't forbidden from trying to steal the sign and communicate it to the batter. However

The cheating aspect is using any outside technology. A runner on second isn't forbidden from trying to steal the sign and communicate it to the batter. However you aren't allowed to have someone watching live video, using binoculars to watch the catcher from the outfield, or any other technologically aided approach to stealing signs. Plus teams often do "encrypt" their signs. That is why you might see a third base coach give 10 different signals. Only one or two of them are needed to convey the instructions. The rest are just used to hide the true signal. However it is now trivially easy to decrypt these signs using machine learning. Here is a video on it.[1] [1] - https://www.youtube.com/watch?v=PmlRbfSavbI https://www.youtube.com/watch?v=PmlRbfSavbI