From what I understood in reading the description for CD is that the metal is case hardened (which is unusual for aluminum; case hardening is usually done on steel). Case hardening is a process by which the metal is chemically treated to have a hardened layer on the outside, and a softer, more ductile core. This is done in automobile engine camshaft lobes, where they are subject to continuous wear on the outside, yet require the ductility of steel.

I'm asuming this process increases sidewall wear against the brakes, but that's about it.