Ask HN: How was this design technique achieved?

I saw this article on ESPN and I was very impressed with the technique that was used. Basically there is a long scrolling single page and in between the "chapters" there is a background image that stays in place while the paragraph scroll along it until its covered.

Here is the url: http://espn.go.com/espn/feature/story/_/id/11717596/is-kobe-bryant-reason-los-angeles-lakers-downfall

Follow up - What is the best way to learn a design technique seen in the wild?