How does Kai'Sa Legend's reaction ability work to add power for casting spells?

Question

Accepted Answer

Ruling: Kai'Sa Legend adds 1 universal power (not a rune) to your rune pool when you tap her. This power can only be used to pay the colored power costs on spell cards, not the numbered generic costs that you normally recycle runes for.

Sequence:
- Declare the spell you want to cast
- Tap the required number of runes for generic costs
- Tap Kai'Sa where you would normally recycle a rune for power
- Recycle any additional runes needed for remaining power costs
- The power from Kai'Sa only lasts for the current turn

Nuances:
- Kai'Sa's ability is a reaction speed ADD ability that can be used any time a card asks for costs to be paid, even when you normally couldn't play a reaction
- Kai'Sa Legend cannot pay power costs for abilities like Accelerate on champion cards, only for spells