954 Hi Monty,
Baring teeth and growling is triggered by an emotional response, therefore the only way to teach it is if you have a dog that naturally shows this behavior in certain situations then start pairing a command to it. This is similar to people who always say “who’s that!!” whenever they see their dog bark at the door (when someone is there) then eventually they can say “who’s that!! (or whatever they happen to pair) and the dog will start barking the same way anticipating that there must be someone at the door. It would have been impossible to teach unless first the dog had the emotional response from someone actually at the door.
Growling and showing teeth is the same idea. You need to have the dog in a situation that naturally triggers that response enough times while pairing it with a command, before you can have the dog do it on that command. Even so, the situation must be similar enough to “trick” the dog into thinking that it may be a situation that may warrant that response when do it as a command only without an actual threat. It is not an easy thing to teach with all dogs because you can not control how all dogs act when threatened. Many of the “protection breeds” will go into a bark and lunge more naturally than a just a growl and teeth since they have a tendency for a quicker trigger “fight drive” when threatened. The more defensive a dog is the easier to teach an command like this.