Fusion Strike is a set of Pokémon Cards, released November 12th, 2021.
There’s not much to say about it; it’s just like any other set. Just like Evolving Skies, Shining Fates for Battle Styles. Just another set of cards.
Fusion Strike cards sometimes have “Single Strike”, and “Rapid Strike” written on the card. These were introduced in the “Battle Styles” set, and it’s just a unique twist to the TGC. You can get the strike cards in the Fusion Strike set, but you can’t get Fusion Strike unique cards from the Battle Styles set. Like J said, when a card says “Fusion Strike”, it’s referring to the middle ground between the two other styles obtainable in Battle Styles.
Choose your attack style in the Pokémon TCG: Sword & Shield—Battle Styles expansion. This expansion introduces an exciting new gameplay mechanic called Battle Styles, introducing two different forms of attack: Single Strike Style and Rapid Strike Style. The former is the Pokémon equivalent of a throwing a knockout punch; the latter is the Pokémon equivalent of a chess game using tactical attacks and building steadily to victory.
You can find a list of possible Fusion Strike cards here.
And lastly, if you’re interested in learning how to play, this is a very good video explains the playing styles.
Source: totally didn’t buy probably $300 worth of Fusion Strike packs, plus links above
If you mean something else, or if I’m forgetting something, please let me know