How does the javascript preloading work?

Question

I don\'t want to know a way to preload images, I found much on the net, but I want to know how it works. How is javascript able to preload images? I mean, I tried a snippet

Answer 1

It just involves making a new DOM image object and setting the src attribute. Nothing clever and AFAIK, it has always worked for me.

Is it possible the second "load" firebug is showing you is it loading it from cache?