Some effects (most notably, the standard effects of Time Passes) discard Power Cards from play because they interact with a specific card that's in play or with cards in play in general. In that case, put the Power Card(s) from play into your discard pile. You can't discard a Power Card from play unless otherwise instructed to do so.
However, the Spirit still needs to pay any costs associated with using the Power, whether that's a mandatory "Cost to Use" (e.g., for Absolute Stasis) or a cost associated with a particular choice (e.g., for Hazards Spread Across the Island). You can pick a Power Card with a Cost to Use that you can't afford; in that case, you fail to use the Power and then Forget the card. Tags: Cost To Use, Resolving Powers, Ethereal Conjunction, Absolute Stasis (Fractured Days Split the Sky), 3 more...
However, the Power Card will be Forgotten before you have a chance to use any Powers in Slow Phase, so it won’t help you hit the Elemental thresholds of those Slow Powers. Tags: Elements, Ethereal Conjunction
If the Stage jumps around (e.g., against the Tsardom of Russia), the it does not matter if the game is currently in Stage III, so long as it has been Stage III at some point earlier in the game. (Note though that the early Stage III Card for the Kingdom of Brandenburg-Prussia does notput the game into Stage III.)
Since the Power Cards don't belong to any Spirit that's still in the game, it's impossible to play them any time other than the Spirit Phase, e.g., with "Spur On With Words of Fire" or "Ethereal Conjunction." Tags: Timing, Second Wave, Tsardom of Russia, Kingdom of Scotland, 4 more...