When you make a Slow Power Fast (via Swiftness of Lightning or similar) gain a 1 Energy rebate.
If a Fast Power Card (or a Power Card made Fast) costs 1 or more Energy, also reduce its printed cost by 1 (e.g., for Repeating it with Gift of Strength or checking when it's played by Rhythmic Power Builds to a Cataclysmic Crescendo).