The effect here is in the Christmas lights—they’re regular lights—and by adding this lens filter, stuff that’s normally blurry goes crazy:
With this in place, the image on the left turns into the image on the right (similar focus distance):
Putting things farther out of focus yields more dramatic results:
As I noted above, this was certainly not my idea—this is just my 10-minute take on it. Be sure to check out the other site’s comments for some really impressive comps.
I put these together in haste so I could impress my wife with my mad skillz (the real photography) but she’d seen the same site as me and pointed me to this spiffy snap-on filter (touché):