Background-Attachment

I came across this cool effect when looking at a website today and wanted to know how it worked. It's a little tough to describe.

When you scroll down the page, you come across multiple panels of content, each with a picture separting them. And as you scroll though, the picture doesn't move.

Scroll down to see what I mean.

The CSS is very simple.
    .panel { font-size: 1.5em; line-height: 1.5em; position: relative; }
    .panel-image, .panel-body { height: 100%; position: relative; }
    .panel-image { 
        background-repeat: no-repeat; 
        background-attachment: fixed; 
        background-position: center;
        background-size:  auto 100%;
        max-height: 100%;
    }
        
CSS is strange in that often when I see a creative web page, I assume it's done with Javascript only to find out it's simple css that has been available for years, and yet is never used. I have to wonder how much of CSS3 I've yet to discover.