A power being "used" or "expended" are phrases that show up in certain important situations, and I don't know of a rules source for the answer. I was hoping that someone here did.
When is a power considered used or expended? Also, can a power be considered used or expended whilst I'm still using it?
Consider these examples:
Ancient Soul is a feat from PHBR: Dragonborn. It states that
Whenever you take damage of the type dealt by your dragon breath (after the damage dealt is reduced by your resistance), you regain the use of your dragon breath if you have already expended it in this encounter.
If you damage yourself with your Dragon Breath, do you regain it? Or have you not expended it yet? (This is the core of the "rebreather" build).
Epic Resurgence, from PHB, allows that
The first time you score a critical hit during an encounter, you regain the use of one encounter attack power of your choice.
Can you regain the power even if you are in the middle of its use?