How does 4chan's thread updater work?

How does 4chan's thread updater work?

In Futaba, their thread update function makes an XMLHttp request and reloads the new page and scrolls you back down to the spot of the screen you were viewing. However, this means that if you had any text in the form field, it'll be gone due to the reloading of the page.

However, in 4chan the form field is untouched when a thread is updated. Whatever it's doing, it seems a lot more fluid and seamless.

So how does it work, any ideas? I tried looking around the source but it's a little too zomgoptimized for me to make sense of it.

>>5
I am a novice trying to understand this particular function that interested me. I am sorry if my lack of expertise is very upsetting. However bad I am at programming, I'm even worse with JavaScript.

This part specifically I don't know what it's about:
    <a href="" data-cmd="update">Update</a>

I've never seen "data-cmd" before like this. I tried searching on Google, but... it doesn't lead anywhere.