Tricktraining Tricktraining topic
About your question.
Some horses will do it if they smell something strange, then you can put a command on it. (I think that's the method of the video, I haven't watched it but read the discription).
An other way is to hold a treat in your hand an hold it in front of your horse's nose. When he will try to take it he moves his lips up, when he does that reward him with your voice and the treat. Do it over and over and over again and he will get that he has to put his lip up. Then you can work on holding his lip up longer=smile
