Like a mod chip, but with a little more circuitry. A little PCB installed (via the ribon connection) between the drive mechanism and the main board of a Sega Saturn that fools the Saturn's security chips into thinking that it was an original Saturn game you put in the ROM drive, not a funnly looking blue and gold one. Doesn't let you play imports AFAIK, so it's only of use to pirates. Of course, with coppies of Burning Rangers costing a hundred quid these days, it's easy to see the temptation.