As Wicklunder already spoke of, the spine of the rod. The name given for the natural flex. I don’t know of a factory made rod that pays any attention to it. Rewinding rods is good for learning how to wrap but I don’t see the performance of the rod changing without realigning the seat. That said, if there is a rod that you find or own that has that sweet action we all search for, it’s worth the trouble to redo. Done many, mostly utilitarian but have finished one the right way and it’s sweet. All these guys will help you as much as you need and have a lot of patience.