I saw an interview with Grandmaster Flash once where he went through the first 10 seconds or so of this song (the non-remixed version I guess) and gave a quick rundown of the techniques he was using, what he was doing. There was an astonishing number of things he had to do. I guess a lot of those techniques have since become common in the DJ's arsenal.

Flash played illegal parties and also worked with rappers such as Kurtis Blow and Lovebug Starski. He formed his own group in the late 1970s, after promptings from Ray Chandler. The initial members were Cowboy (Keith Wiggins), Melle Mel (Melvin Glover) and Kid(d) Creole (Nathaniel Glover) making Grandmaster Flash & the 3 MCs. Two other rappers briefly joined, but they were replaced more permanently by Rahiem (Guy Todd Williams, previously in the Funky Four) and Scorpio (Eddie Morris, also used the name Mr. Ness) to create Grandmaster Flash & the Furious Five. Soon gaining recognition for their skillful raps, Grandmaster Flash & the Furious Five pioneered MCing, freestyle battles, and invented some of the staple phrases in MCing. They performed at Disco Fever in the Bronx beginning in 1978.

http://en.wikipedia.org/wiki/The_Message_%28song%29

here's a replacement for your grandmaster flash video that no longer works!

An incredible mashup of Shaun Of The Dead zombie film & Grandmaster Flash - White Lines...It fits beautifully! (here's one of the White Lines references in the film, via a funny dirty-NSFW/clean-Funky comparison clip!)