The link you included (https://www.binwang.me/2022-10-29-RSS-Brain-Yet-Another-RSS-...) has some details, but I guess you are right it's not obvious enough.

The way RSS Brain knows upvotes is by leveraging customized fields in ATOM feed. It will parse the tag <*:upvotes>, <*:downvotes> in each of the article and compute score from there. That means the RSS feed should support those fields in order to let RSSBrain know the upvotes. But the protocol is open so any RSS feed an RSS reader can use those if they think this is a good idea.

So it can only get exiting upvotes from the site. It has no way to upvote when there is no such feature in the original site. However, you can have a customized RSS feed to use upvotes in a different way: for example, for a news website, the position and size of an article usually means the "upvotes" from the editors. Which you can give some weights based on the position and generate a RSS feed that use those weights as "upvotes".

I did two things to make it easier at the beginning:

1. When you subscribe to HackerNews or a subreddit in RSS Brain (that means you paste HackerNews/subreddit URL directly to RSS Brain to subscribe), it will try to parse the content by itself (using some APIs), instead of trying to find a RSS feed. It's not a standard way, but just a way to make it easier while no RSS feeds have those fields yet.

2. I pushed a PR to a very popular RSS generator: RSSHub, so that you can easily add those fields (the doc: https://docs.rsshub.app/en/joinus/quick-start.html#submit-ne...) when create a RSS feed using RSSHub. I've updated its HackerNews RSS feed to generate upvotes fields as well.

Feeds (RSS, ATOM etc) often times contain only a small subset of recent items, like 10-15 of them. This means that if you limit yourself only to the feed themselves, you will lose track of upvotes at some point, for example after a day or two past the publication time of a given item.

Obviously this is something you have thought through, so I assume you just treat item disappearing from the feed as some form of indication it's not "trending" enough from the source point of view. Am I right?

That’s correct. If the post is not hot anymore, continue monitoring new votes wouldn’t have much impact on the sorting, at least not the top ones. And because for sites like HackerNews and Reddit, once it’s not trending enough, it’s rare the upvotes continue to increase a lot. So this is a trade off I think makes sense considering the nature of RSS feed.

Curious about your last question too. I assumed this sorting only works for websites that do have voting (and the count is included in the feed).