I don't know what it is, but my productivity leaped when I got a standing desk.
Now I send out dozens of slashdot comments a day!
But I kid.
Really. Much more productive doing actual work.
Anytime I need to do real thinking, I've historically gotten the urge to walk around. Much like the Sundance Kid, "I'm better when I move." Or that's what I thought. Now I think maybe standing with minimal movement is enough.