Way back in 90′s Rare and Nintendo were part of an unstoppable team that seemed to have the Midas touch, every game that was made during their tenure together sold by the bucket load. In 1998 Rare produced what would become the pinnacle of their partnership Banjo-Kazooie. So good was B-K that it was immediately and favourably compared to Mario 64, some commentators went as far as to say it was better, but ultimately Mario still retains his crown.
Hidden deep within the Banjo-Kazooie levels were a selection of eggs and a key that were unobtainable without using an action replay (or similar). At the time their function remained a mystery and while Rare teased that these items would play a part in the game’s sequel, nothing more was mentioned until Banjo-Tooie was released. When this happened the Stop n Swop feature was included but failed to perform the promised backward compatibility.
The answers about Stop n Swop remained elusive until in 2008 when MTV spoke with Salvatore Fileccia and fans were awarded the first direct answer to the whole mess.
Fileccia cited that the abandonment of Stop ‘N’ Swop was due to revisions made to the Nintendo 64 circuitry. He stated that older versions of the system would have given the player 10 seconds to swap cartridges, while newer iterations of the console reduced this time to one second.
Finally after all those years of wondering the Stop n Swop feature has been implemented between Banjo-Kazooie and Banjo-Tooie, when Banjo-Tooie is released on XBLA. If you would like to know what the fruits of your labours will give you, then the guys a The Rare Witch Project have played Tooie to find out. I would say this video contains spoilers, but only in the sense of what you earn and not how to find the items for the Stop n Swop.
Source Rare Witch Project and Wikipedia