[EricH]

And not only do you get situations like Ethulin described, but then there's where a whole team starts yelling at the drivers to do something. Of course, the refs and drivers have a hard time hearing it over the other teams and music and crashing of robots, and this is extremely common, so they will likely let it slide.

Back to the original question, I think the general rule of thumb is, no talking to the coach during the match unless you are a driver, and definitely not by radio/cell phone/flashlight(Morse code and equivalents)/hand signals. Yelling at him is likely not a good idea because he may get distracted, and you don't want that, do you?