Ah. Hitting with the secondary attack recharges the One Sword attack. You can keep using it as long the secondary attack keeps hitting (which requires that the primary attack keeps hitting as well, of course).
round 1: attack with the One Sword, hit. make secondary attack, hit.
the encounter power would be expended but is recharged because the secondary attack hit.
round 2: attack with the One Sword, miss. the encounter power is expended.
alternate round 2 #1: attack with the One Sword, hit. make secondary attack, miss. the encounter power is expended.
alternate round 2 #2: attack with the One Sword, hit. make secondary attack, hit. the encounter power would be expended but is recharged.
