It's very easy to get the message to the batter. A few years ago, I was coaching a teenage baseball team and while in the 3rd base coaches box, I could read the catcher's signs. I used keywords to tell the batter what was coming, either the word "smoke" or "wait". I might say, Get your pitch and smoke it, and he'd know a fastball was coming. Or I would say, "Wait on your pitch." and he'd know it was offspeed. Worked like a charm.