Stop 'N' Swop is a system for connecting the games Banjo-Kazooie and Banjo-Tooie. At the end of Banjo-Kazooie if the player collected all the "Jiggies" they would be greeted to footage of new areas with new items and was said the secret to unlocking them would appear in the sequel. Hacking of the game later revealed 7 of these secret items as opposed to the 3 shown in the bonus footage.
When Banjo-Tooie came along the correct way of unlocking the items was never revealed and instead the three items shown in the footage and one of the ones not shown were found in game and unlocked bonuses.
What is known about the hardware system behind it reveals that the items in Banjo-Kazooie would probably been unlocked by pulling the Banjo-Tooie cartridge out of the Nintendo 64 and replacing it with Banjo-Kazooie which would unlock the items. Once collected they would be transferred to Banjo-Tooie in a similar way. However hardware changes to the N64 and fears of damaged consoles and games by Nintendo forced the feature to be cancelled.
Now that both games have been ported the XBox 360 the feature has been reinstated however it is now preformed by software and not hardware changes. To unlock the items in Banjo-Kazooie only requires to player to also have Banjo-Tooie on the same system or a save file of the new Banjo-Kazooie game, Nuts & Bolts. Collecting the items automatically transfers them to Banjo-Tooie and Nuts & Bolts.