Oh here's the story. Supposedly you were supposed to do it with the power off but while the N64 still retained the memory. Buuuuut:
"In 2008 MTV conducted an interview with Salvatore Fileccia, lead software engineer at Rare. 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.[6]"
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Thank you, Eddie Guerrero.
http://www.wwe.com/f/styles/photo_large/public/photo/image/2012/04/33_Moscow_04112012jg_02128.jpg