According to Ed Rivadavia of the AllMusic Guide writing about the original Montrose recording: "...the band and Warner Bros. staff producer Ted Templeman weren't terribly impressed with what they'd managed to record at the outset. The song was missing a certain something to make it stand out, and according to guitarist Ronnie Montrose, he finally stumbled upon that 'something' while messing around in the studio with a slide and fuzz box one day. With his axe in open D tuning, Montrose began improvising what became the song's trademark motorcycle rev-up riff, while Templeman and engineer Don Landee flailed frantically at him to keep going while they scrambled to start the tape rolling. Sure enough, the nearly out-of-control roaring engine effect served not only as the song's intro, but became its most distinctive and recognizable feature, and has often been replicated since." Including on the live version, which runs to around 7 minutes 4 seconds.
Alexander Baron - London, England, for above 2