How to Lux Crownguard ability?

Based on the card data for Lux, Crownguard, here is how her ability works:

Ability Breakdown

• Text: exhaust : [Reaction] — [Add] 2 energy . Use only to play spells. (Abilities that add resources can't be reacted to.)
• Cost: You must exhaust Lux to activate this ability.
• Effect: You gain 2 energy that can only be spent to play spells.

Key Rules & Nuances

1. Floating Energy: When you activate this ability, the 2 energy enters your "Rune Pool" as floating energy. You do not need to spend it immediately; you can use it later in the turn.
2. Spending: This energy can be spent on one spell or split across multiple spells. Any unused floating energy disappears when your rune pool empties (at the draw step and at the end of your turn).
3. Timing: Because this ability has the [Reaction] tag and the [Add] tag, it can be activated at any time you have priority, including in response to an opponent's spell.
4. Unreactable: As noted on the card, abilities that add resources (with the [Add] tag) resolve immediately and cannot be reacted to.
5. Usage Restriction: This energy cannot be used to pay for effect costs (like "counter a spell unless its controller pays 2") or unit abilities; it is strictly for playing spells.
6. Exhaustion: Since the ability requires you to exhaust Lux, you cannot use it if she is already exhausted. If an opponent uses an effect to exhaust Lux, you can respond by activating her ability before the exhaustion effect resolves, provided you have priority.

Disclaimer: This interpretation is based on the card text and standard Riftbound mechanics. If you have a specific scenario involving other cards, please let me know!