[Justin]

actually... Most of the songs u can play them using playback which u can get lots of meanings... Not only rock-metal music I even played in a party some english RNB music where I used the reverse option.. i was shock that many & I mean many songs pretend to be this way by mistake sometimes it lose the real meaning but still u can understand it... I swear but there's some bands who works on doing it azdan... but Im not sure that Led Zappelin did it by mistake