Ask HN: How does Reddits' real-time upvote counter work?

Example gif: https://i.imgur.com/3rh9tz5.gif

Reddit recently introduced counters on all posts that update "in real time" the upvote and comment counter for any given post.

The fact that there are multiple posts on a single page with millions of active users at any given moment, results in tens millions of requests being sent back and fourth "just" to show a cool looking counter component.

This simple looking counter must have some level of complexity behind it, which made me curious about how it works.

I've been trying to reverse engineer it with Fiddler for the last 30 minutes with no success, so I figured that I should ask here.

How does it work from a technical point of view?