I think this is one of those times where a viable in game mechanic just cant be explained without being cheesy. Taken at how its written your literally stopping and starting the rage ability every 6 seconds which doesnt flow well in a story mechanic.
In this case i think its best to focus on the end result. The barbarian no longer suffers from fatigue as his rage offers him a wellspring of furious energy. Once exhausting feats of strength are no longer as taxing as they once were. This allows the barbarian to use his once a rage powers now once a round.
The explanation here doesnt take the rage cycling mechanic into respect at all but the result is exactly the same.
On a side note i think if paizo was worried about an over balance on rage cycling we likely would have heard or seen errata by now. At the very least they wouldnt have made 3 other viable ways to combat fatigue in this manner within actual game content.
