Ask HN: What is “vouch”?

I see a new Vouch button has appeared under /newest but I couldn't find any information it. It appears to override the automatic flagging system (and presumably is only available to users with high karma).

Can anyone point to more information, discussion about how it works, and guidelines about when it should be used?

I used Vouch on https://news.ycombinator.com/item?id=11589331 which seems to have been unfairly marked Dead. Apologies if I'm doing it wrong.