By compelling you to avoid saying something. If you have a prearranged signal, you don't get to say "I'm just keeping my silence" any more: You are triggering a signal.
As far as I'm aware, this hasn't been tested in court. However, given that courts have been pretty silent on the idea of secret laws and compulsory silences, if it did get there, I'd be shocked that courts didn't rule this way.
By compelling you to lie? When and where was that "pretty well established"?