The calling protocol in bridge is strict, you aren't allowed to use hidden signals to say that although you call spades you really want no trumps. You can change your call depending on your partner's call but not because he kicked you under the table.
500 by comparison as played in the services is Rafferty's rules. Say you have the joker and little else you call "six birds" instead of six no trumps. You won't start a fight because everyone else knows what you called.
