Anyone who has seen competitive sheep herding knows that dogs have an intense interest in picking up even the slightest cue from their handlers.
And these cues are “broad band” as well, which means gestures, facial movements, sounds, and smells.
Once I have whistled my dog to a turn-stop-sit and she is sitting out in the field awaiting my cast, if I so much as twitch she is OFF. I have to stand perfectly still before I give my cast. She has learned to watch my head tilt in the direction I am going to cast, but wait for my hand signal. But that took some doing.