By bong @ HKCC.
Checks the screen, touchscreen, buttons, speaker and microphone.
A / B / X / Y: Test both speakers, change color
Up / Down / Select: Switch screens
Left / Right: Change color
Touch screen: Shows stylus position, change color
L: Test left speaker，change color
R: Test right speaker，change color
Start: Toggle microphone check
Blow into microphone: change color continuously
Official Homepage: Original forum thread (Chinese)