It was a bunch of good artists doing a verse over a Duke Westlake beat and one would call out the next at the end of the song and when they heard it they'd make a vid and keep it going. Just youtube search Duke Westlake cypher.
Intuition
[video=youtube;gPjuHKfLPdo]http://www.youtube.com/watch?feature=player_detailpage&v=gPjuHKfLPdo[/video]
2 of my other favorites are
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Palmer Squares
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Watsky
The last guy Watsky is the one who put it to Gambino then it died because Gambino never answered. Say what you want about either artist Watsky did this cypher justice. It's been 2 years though I doubt Gambino is gonna respond.