when you scuff up a perfectly good smooth cylinder with a rude sort of carborundum device, you are in fact only putting scratches on it, giving us a new, larger diameter at their base - sort of like root and pitch diameters in a nut, say. And when you put it back together, the new rings and this rough surface proceed to grind each other down until smoothness is re-attained. By this time, needless to say, you have larger ring gaps, due to more ring wear, and more piston clearance, since the cylinder has been taken down now to the base diameter of the honing scratches. And the valuable metal so removed has been converted into grinding compound circulating in your oil, so you have also reduced the size of your piston, and contributed significantly to wear of all the other parts as well....Although it is true that *chrome* rings are so hard and smooth that it can take from a long time to forever for the cylinder to wear them to perfect fit, this is *not* the case for Cast Iron rings. In fact, that's the secret. Chrome rings only in rough (honed) bores. And hone bores only when you have to, ie as the final "finish" of a bore job. Since the boring bar tool in effect runs a screw thread down the bore in its journey toward the land of largerness, it is necessary to do a final "polish" with a somewhat cruder tool - the hone. This is the *only* reason to put a hone in a cylinder. And when it's done, the machinist has purposely left some extra meat in there for the hone to finish to final diameter. And the final diameter is a tad small as well, in anticipation of the initially rapid wear that is inevitable as the new rings and the scratchy bore get acquainted. This is also the reason for very careful cleaning of said bores before assembly, and for very frequent oil and filter changes, until the motor has stopped manufacturing grinding compound during the break-in process....
Were they the same guys who said to thrash a motor to break it in? 
