Each bubble represents a stat, Vocal, Dance, and Visual (I can't remember the order though). What you want to do is hit any one button on beat with the song. When you hit a button, you'll get points. The value of that button's point multiplier will lower a little bit at the same time. When you hit a different button, the value will go back up. So basically constantly hitting the same button will keep decreasing its value while increasing the other buttons' value (up to a certain point). Therefore you want to switch up which button you press (on beat as best you can) based on each button's point value at that point in time and based on how high you trained each stat, too. There's also this amulet business, but it's not a big deal in the demo.
I don't know Japanese but I know how to play the game, so I'm sure someone else can explain it better than I can.
