There's some material science answer to that, when I was still in school I could maybe have rattled it off, I sure as heck couldn't do it now. In general it just makes the metal stronger, this has been gaining increasing popularity, at least in the aftermarket VW world, Aircooled Technologies in Georgia somewhere started doing this about 3 or 4 years ago. It was only $50, so I kinda figured "what the heck?"
