A solid copper wire that is bent repeatedly work hardens at the bend. The grain structure changes, the metal becomes brittle at exactly the point that keeps flexing, and it eventually cracks. No amount of copper purity prevents this; it is what happens to drawn metal under cyclic strain.

Tinsel construction changes the geometry of the problem. Copper is rolled into a thin foil and wound helically around a textile core. When the conductor bends, the core takes the tensile and compressive strain and the foil follows it, sliding slightly rather than being stretched. The copper is never asked to absorb the bend.

The practical result is flex counts that a drawn strand cannot approach, which is why tinsel is used in headphone cords, speaker leads and anything that is coiled and uncoiled daily. The trade-off is conductivity per unit diameter: a tinsel conductor is bulkier than solid wire of the same resistance.

Because the copper is a foil rather than a drawn strand, resistance and outer diameter can be traded against each other by changing foil width and lay. A fixed wire gauge cannot do that, which is why tinsel is specified by target resistance and diameter rather than by gauge.