If you look on the gamecube controler, look under the grey analog stick.
There will be arrows pointing up, down, left and right.
In order for this cheat to work, you must press (in this order): "up, up, down, down, left, right, left, right, B and A.

Ps: sometimes it does not work because of two arrows pressed at once!
SO I WARN YOU, WATCH OUT!