This is neither a cheat, nor a hint, so I really didn't know where to poet it. I decided that it was closer to a hint so , her you go.
I know that this works for Faust. You will have to do some expermenting to find out if it works for other characters or not. You MUST first unlock VS mode.
FIrst, load your game in story mode. Then, get into a fight with Yoh being one of the characters in the fight. Stay in it untill one character, even Yoh, uses a special. This can be any special. Then you have to loose. When it askes you if you want to continue, pick no. This will take you to the main menu. From here, pick VS mode. You have to have another controller and someone to play. Now, one of you has to pick Faust (only because that is the only person that I know this works with). The other person can be anyone they want. In this battle, use the Boneyard Crush, and make sure that you win the battle. Then, in the next battle, pick Faust again. When the characters are getting their Spirits ready, you have a 50/50 chance of hearing Faust saying Boneyard Crush in a very creapy voice. If you listen carefully enough, you will also hear the skeletons (spelling) comming out of the ground. My brother and sister found this out and I didn't believe them untill I saw it. It was sooooo cool.