"B" changes (scrolls) the shapes to match the one dropping down the screen.
Once you've got a match, Press A to send it to the left if it is dropping down the left hand side of the screen and press B to send it to the right, if it is dropping down the right hand side of the screen.
It gets faster and faster as you get further in the game
The "B" button changes the shape in the bottom middle. Look at the left or right. Quickly press "B" to change to the shape which is shown.
If that shape is on the left, press the "A" button. If the shape is on the right, press the "C" button. They get faster every time you blast a shape successfully, so be careful .