DonM What makes a shaft smooth? Not a trick question!
In Nippon's case it's quality control that's where it starts. They manufacture the best quality available in steel. True Temper's quality in Amory, MS is some of the worst. Smooth can mean different things to different golfers. Shaft fit = club head speed, transition time, release early, mid, late. The transition time and release are what separates golfers into shaft needs and shaft profiles they fit. Heft separates them further with shaft weight/head weight into playing length.
Example a lot of seniors and ladies are fit into too light a shaft weight and low heft match which makes the club "feel" too light and they can't keep club on plane.