Content fade-in as user scrolls browser window

I'm discovering more websites that perform this visual trick where as you scroll down the browser window, the content fades into view.

I'm wondering how this trick is done and (more importantly) why? I'm guessing th开发者_开发技巧at for exceptionally long pages, the content isn't fetched until you actually reveal that div, image, whatever therefore saving bandwidth?

Lastly, what is this technique called?

Here's a link that triggered my curiosity: http://blog.insicdesigns.com/2010/09/showcase-of-games-developed-using-html5-canvas/

Thanks!

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http://www.appelsiini.net/projects/lazyload - that sort of thing?