Bolton is an ancient house. They were bitter rivals to the Starks back when the Starks were Kings in the North. Back when magic was common place, and the Starks were wargs. How did House Bolton survive this feud, against an enemy with superior man power and magic? Recall that the Boltons were known to have worn the skins of their enemies as cloaks, even having a few Stark skins back at the Dreadfort. Well what if this was the Bolton ace in the hole? They have been flaying people since the beginning of written history. Is it such a stretch that they would know some magic art pertaining to human skin? I think this is the ultimate survival technique. About to be captured by Starks? Okay, let me just don my Lord of Winterfell skinsuit and tell them to fuck off! Perhaps what looked like quick thinking on Ramsay's part when he assumed Reek's identity was actually Bolton instinct.