How to teach my dogs how to snarl on command?

All my dogs know how to bark on command and be quite just by using hand signals as with the basic tricks such as sit, lay down, shake, roll over. I just thought that getting them to snarl and show their teeth on command would be an amazing trick to teach them. Im hoping to be able to do by a signal instead of verbal command. Anyway any advice will be welcomed and appriciated! :)