[FrankJ]

Originally Posted by VioletElizabeth

To use the words I think in, my best guess as to what that means is you whack a thing so that each little bit of the surface wants to push itself apart/press against its own neighbor, which prevents cracks because cracks get pushed back together by this pushing.

Sorry if that was too silly, thinking in simple words helps me.

Not a bad way of thinking of it. Shot peening parts does something similar & is also specified when fatigue cracking might be an issue. (not for threads so i am drifting off topic) Rolling the thread also gives is a better surface finish than a cutting tool that leaves little micro cracks the form stress risers.