When you submit the form#filters_form, it requests the response in the background, replaces the existing contents of #article_blocks with the #article_blocks from the response, and updates the url with the params given.
Oh, and if it fails, it redirects you to the form url, so that you're not left in a broken ajax state.
So:
- No fancy ajax logic server-side
- No fancy ajax logic client-side
- Just the bit of content you wanted gets updated
Disclaimer: Tweaks would be needed for POST forms