Take this post for instance, the link to the upvote is https://news.ycombinator.com/vote?id=48106316&how=up&auth=26...

If you change `how=up` to `how=down` it downvotes the post.

I tried it once, didn't change the vote count, but I issued the inverse operation with `how=un` and the vote count went up, so I'm guessing someone upvoted at the same time I downvoted. That or it doesn't really work like it says it does, but it does respond with a 301 followed by an OK, so I think this works.

This.