Maybe I'm missing something, but I don't see how the One Sword attack is confusing.Yes, it is quite confusing.
The way we ruled it during the game is that I only got the secondary attack on my first turn, and the hit lasted until the end of my next round.
What was "sustained" was that I could continue using the primary attack of the sword for the entire encounter (because I hit with my secondary attack).
If you hit the target, you get a secondary attack vs the same target.
If the secondary attack hits, the target is dazed until the end of your next turn, and the power isn't expended (allowing you to use it again in the same encounter).
If the secondary attack misses, the power is expended for the encounter, as normal.