I went to a Nintendo event during a trip to New York in 2004. I brought my copy of Pokemon Ruby with me to play, and, while walking through the Nintendo Centre, saw an event station. You hook it up to a machine similarly to if you were trading to a friend.
The machine was basically a self-controlled GBA, and when you turned on the game with this connected, you would go through a phase similar to that of a trade, except only half of the animation is shown, since you don't have to trade anything yourself.
The machines might've changed around a little through the GBA years, but at that time, that's how it worked. =]