Katakuri lost his composure, unable to use COO at all, a moment which Luffy used to deal as much damage as possible to Katakuri. But you're right with one thing: FS is his most special ability, just like King's rare ability and Marco's regen. In Kata's case, when he can't use FS, his other abilities can't work either since Katakuri would be usually not by himself when it comes to his mindset.
Losing composure or getting highly distracted mid-battles are an important factor to twist the fight in a different direction. Look how quickly Kizaru got rid of Marco after the latter was distracted too.