Charlie is right. They use new, highly polished dies and highly polished and extremely clean planchets, as well as hard multiple strikes. The results are, hopefully, a "frosty' relief against a perfectly polished background. When everything works, the coins are almost magical. The relief seems to float over the surface. Play with them, put them in the ground, and they go south in a hurry. An expert may be able to tell that the coin was once a proof by looking at the surface and how the scratches look but it no longer holds the true "proof" status. It would be a proof coin in a grade less than MS-60. Sort of a circulated proof. It would be interesting to see how they would actually grade one if someone found it and had it slabbed.
Daryl