Stop ‘N’ Swop is a hidden feature in Banjo-Kazooie. Rare announced that special areas and items in the game could only be reached by completing certain tasks in its sequel, Banjo-Tooie. BT was released in 2000 and offered a way to retrieve the items but without the need to acquire them in Banjo-Kazooie. It was later discovered that Banjo-Kazooie contains seven special items which can be accessed using in-game cheat codes or by using a cheat cartridge. These items would then be viewable in a menu entitled “Stop ‘N’ Swop”. There was also an inaccessible Ice Key shown in the sequence, which induced gamers to search for a way to get it.
In 2004, a patent filed by Rare was published which suggests that Stop ‘N’ Swop involved swapping cartridges with the power off to transfer data. The information would be momentarily retained by utilizing the Rambus memory in the Nintendo 64. As a result of changes done to the Nintendo 64 systems produced in 1999, the system could no longer do this effectively. [Info from Wikipedia]
At the RareWitchProject website they are working from many years to research the mistery behind the “Stop ‘n’ Swop” and a couple of days ago they have found another interesting proof of the Banjo kazooie / Donkey Kon 64 connection. As SubDrag says in his latest news: “DK64 has always had a mysterious connection to Stop ‘n’ Swop. In our DK64/BK SNS Special, we broke a revolutionary news story about Stop ‘n’ Swop originally not being between BK and BT, but BK and DK64!”. Runehero has find a way to activate the Ice Key in DK64, that was meant to open a secret door in the game!
For more info you can read the original topic in the RWP Forum! Huge props to Runehero and all the RWP users :)