(I mention this just because in the periodic NoScript debates that show up here, I think people don't believe me when I say for every site it hurts, there's another site it silently improves.)

If I had to guess, it's to entice people to scroll down. I don't think it's done perfectly, but I do think the execution of this idea is better on http://goodui.org/howtoabtest.html

The fact that there is something there, and it's kind of hidden (at least on my screen) then there is this natural curiosity built up..

It's important to have good content to draw people downwards, but when you combine good content with small psychological tricks-- it might just pay off. The guy has 17K email subscribers and that is a lot of people who do trust him

That, and the general focus of the content itself, put across a message of conversion-centric design rather than human-centric design. Ignoring the way a person feels while interacting your design just to increase signup conversions. Not nice.

As I scroll, it keeps reappearing right in the middle of the content, then flashes away after 2 seconds

When I got to the end, it was nigh impossible to actually get the footer to show (or whatever the sign up box was) I finally got to enter my name but then going to [Next] on iPad just made the whole thing disappear and I couldnt get it back.

I like their ideas, really dislike their execution, that was a frustrating experience.

Edit: It's a good idea, great marketing idea, in fact. But when you're advertising ideas to improve UI, I will be quicker to judge...

Are there browsers where virtual pixels are not automatically scaled on retina displays?

I actually didn't see the footer at all, shows how my brain learned to keep me sane.