Go to last read post — how does it work?

How does this forum's software know what the last post I was reading was?

One can click a little icon on a thread that beams you directly into a thread and down the page to where you left off. Near as I can tell this happens by magic.

I once looked for some ajaxy thing that updates the server with your current progress through a page. I could not find anything like it.

Any clues?