Also the harder metals, steel, bronze etc, will cause excessive shock waves through the stone, and can cause the stone to fracture, or shatter internally, and when you get to that spot, it falls apart. Also the steel will tend to bounce back, where as the copper will tend not to. The Copper is softer, and will absorb some of the shock. It will also deform, and actually grab the prepared surface, and help to pull the flake off on percussions. On pressure flaking it does somewhat the same thing, except that you are building up the force instead applying all at once as on percussion. I hope this makes sense.
It is my "theory", from what I have read, and from trying steel, and brass, and having the rock crumble, and finding internal fractures.
Wayne