They were using video feed of the catcher’s signs in real time and then signaling the batter while he was in the box if off speed or fastball was coming. The Astro’s 2017 chase rate on breaking balls was supposedly pretty low at home.
Your point about changing the codes is right on. Just how dumb are catchers and pitchers? It is down to stubbornness, laziness or dumbness that all catcher’s signs don’t have changing pre-decided indicators that change the signs or the sign order with today’s technology. You base it on inning number or the score or number of outs or anything else that changes. It would completely foil any sort of sign stealing, legitimate or via technological device.
Freegards
The World Champion Nationals were changing signals constantly.