The premise is simple: take one NES cart, one NES, empty the working-bits from the NES, empty the NES cart, put the working-bits from the NES and squeeze them into the NES cart. Insert actual NES cart into NES cart NES, plug into TV, play Mario. Easy. It's the slimline NES we would have had if old Nintendo ever pulled a new Nintendo and released a slimmer, smaller NES, then released it in limited-edition, themed variants.
Fami-card [Kotmoi, via technabob]