Now that I have the game, I can't duplicate the trip consistently. It almost seems random, but it always happens when suddenly tapping forward. I've tried just tapping back and forth rapidly in training mode and not tripping at all. But other times, I'll just tap Forward+B and suddenly trip.
Oh, and you can't cancel it. It serves no useful purpose, it seems to be either a punishment for doing something wrong, or its entirely random and just another one of Sakurai's attempts to inject randomness into the game. I've searched several Smash boards and found no one who can explain how to reliably do it (and no, its not just from doing a failed dash dance or moonwalk, most of the times when I have tripped I wasn't even dashing, I was just standing there and tapped forward+A or forward+B and fell over).
At this point, rather than being upset about Gimpy's little joke with the name, I'm upset this "move" exists at all. All its done so far is irritate several people in our smash group who keep accidentally tripping randomly with no idea why. I wish Sakurai would update the Dojo with an explanation of tripping and, rather than how to do it, how NOT to do it, because you can't cancel it and all it does is prevent you from doing the move you intended to do. I almost wonder if it was put in as a way to spite tournament players trying to get rid of random elements.
