The smile is a simple trick that is great to use when you are first starting clicker training. It helps establish the association of the click with a reward, and it helps build the horse's (and your!) confidence with clicker training. Initially, reward any attempt to lift the lip, even if it is to grab at the treat in your hand. Then, once they are consistently lifting their nose, you can focus on keeping them out of your space and not being grabby. Of course, if your horse is being aggressive, you will want to correct from the beginning so you don't teach bad habits.