There's a piece of hair draped over the top of the knot to create a retro curve of hair down her forehead.
And when you get in super close to the "bun" part...
...it looks like there's some very, very loose braiding going on there, or at least some twisting. But knowing Lauren's love of braids, I would imagine it probably really is a plait of some type. It's just been pinned up into a ball beside her ear.
Anyone want to try it out for us? If so, report back!
Related: The Most Charming Hairstyle Ever Is Back (This Time On Lauren Conrad)