A+B+Start+Select=Soft Reset.
Press all those buttons at once. It skips the "Nintendo" screen and takes just a little less time then turning your game on and off.
Soft Resetting is like you turn on and off the GBA, but it's mostly used to get shinies like starters, Rayquaza, Groudon and so. People use it because it makes the GBA turn on faster...
