Assuming you're talking about the debug bios for the NGH system, this is the deal:
1) Your machine will always boot up in JPN mode, whether it is a JPN system or not.
2) To enter the debug menu, you just press select + C. Then you can enter whatever mode you want (home, mvs, etc.)
You can mess with the violence settings, change the number of lives, length of rounds, etc.
Sometimes the MVS mode isn't available on some of the newer games (it's a copy pretection issue, I believe).
All in all, it's very helpful if you want English text and blood/boobs.
A must have for your NGH system in my opinion.