Lol, I was asking the same question six months ago. Well...here's what it is and how to do it. Mind you, don't do it if you want to keep your tama, coz you can't download afterwoods! Also it doesn't worked on a debug tama.
Go to the main screen, then hold down all the three buttons. Whilst holding them, use a sharp object to press the reset button. Then press A, B, and then C. This will do the pixels and icons test. Then press A twice, and put your normal tama on "Connect". If you do it right, the normal tama will either play a game or receive a present from the egg (the rom test tama).
The rom test is used in the factory to make sure the infrared is working properly, but it's a useful and entertaining trick to do even for normal tama owners. Just make sure the tama you're gonna do it on doesn't have anything you want to keep (points, items, or generations). Have fun!
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