User blog comment:Kaido King of the Beasts/Chapter 879: Mirror Battle in Mirro-World/@comment-27930128-20170922112820/@comment-24850639-20170922185451

This fight is basically gonna be a lesson of how Luffy can use his Devil Fruit better. Katakuri is content to use his observation haki and just wait for Luffy to attack, rather than going on the offensive himself. Once Luffy throws something at him that gets him out of defense mode (Gear 4th), he'll need to actually start taking the initiative, and that's where Luffy will get an opportunity to truly evaluate his opponent's strengths & weaknesses. Luffy can't learn anything valuable about Katakuri's techniques if he can just play defense and copy everything he does, so he's gotta knock him out of that comfort zone.

Hopefully all the unexpected applications of Mochi will get Luffy thinking about all his untapped potential. Getting chills remembering the first time Luffy activated Gear Second, and explained how much he learned from just watching CP9. If Katakuri can learn from Luffy, I see no reason why vice versa can't be true.