During the Galaga demonstration, wait until the Galaga Commander uses the tractor beam. Before it sucks up the ship, press fire and kill it. You now have total control and can play the sample level. However, finish quick or an error screen will come up. The game will function normally though.
Please do the following:
For a faster Pac-man/Ms Pac-man, or to shoot faster in Galaga (or to toggle the speed cheats OFF), insert coins, then enter the following sequence: