This is one of my favorite knitting blogs. I have learned so much from the Techknitter! She's from Wisconsin and has two posts just on the subject of "bulletproof" mittens for northern climates. The thrums on these are added AFTER the mittens have been knit. I saw something similar to this adding thrums to the inside with a crochet hook. This may be what she's doing here. You have to buy her patterns, but they're only a few dollars.