It works by establishing a websocket connection to the Hacker News Firebase database, to receive updates every time the HN server updates Firebase, which is about once every 30 seconds. This is very efficient, putting no load on HN's server and using minimal bandwidth.

To make the feed continuous despite the delay, it waits to display each item until exactly 30 seconds before displaying it. I think this is a fair tradeoff, it gives you a sense of how active HN is. For comparision, there are something like 6-7 thousand tweets every second.

Official HN Firebase API: https://github.com/HackerNews/API

Source code: https://github.com/jerbear2008/hn-live/blob/main/index.html