The slang usuage of "Flow" in Hip-Hop changed thru out the years. its meaning is constantly evolving thru out the generations. but some people today still use the old terms to express certain things cuz theres no words for what they are trying to say. but here are when they came around... the dates maybe off a year or two. (1989 to 1996) To rap fast in a straight line and to roll your tongue to make it sound even faster. back in the early 90's this technique was the newest thing to do and hardly heard in a Rap song. (1996 to 2005) To "Rap." at this time it was simplified and bastardized to its simplist meaning. (2005 to now) To Rap with a Unique Melodic Rap Style.