All this is unofficial, but based on my own observations:
You can never down-vote a submission. When you have enough karma you can:
* Down-vote comments
* Flag comments
* Flag submissions
Flags on submissions affect their ranking in a manner similar to down-voting, except that enough flags can result in a submission being deleted entirely.
Flags never delete a submission entirely. We don't delete submissions unless users ask us to (and have a good reason). Everything else remains visible with 'showdead' turned on in your profile.
Or any of the others who work on the code. However, even jacquesm[0] - third on the leader board[1] - says that to the best of his knowledge one cannot downvote articles:
There are no downvotes on stories, unless you consider flagging a kind of story downvote. That's a legitimate way to look at it, since it answers the complaint upthread [1]: if you think that a story is not appropriate for HN, that's what flagging is for.
You can never down-vote a submission. When you have enough karma you can:
* Down-vote comments
* Flag comments
* Flag submissions
Flags on submissions affect their ranking in a manner similar to down-voting, except that enough flags can result in a submission being deleted entirely.