There was a super simple (MS Paint level) graphic I saw once, probably on this forum, which made the process very clear. Basically, when a small hole occurs, for whatever reason, in the chrome, the steel below oxidizes. This results in the rust blooming up through the hole in the chrome. When you use the aluminum foil on the chrome, the foil is harder than the rust but softer than the chrome. The rust is removed while the chrome is unaffected.