I read somewhere about how NGC's restoration service removes a "microscopic" layer of metal yet still leaves the luster intact.
I think it means they carefully dip the coins in chemicals or dips made for coins. Dips remove some metal. The dip changes color from the metal. If you dip CN coins then copper in the same dip you can see the silver color on the copper or vise versa. (so they only do one metal type the change dip)
If a coin is over dipped then the luster is messed up but if they do it right it may improve the coin, they know how long to dip and they rinse well so "most" coins survive their restoration without being messed up.