[MattBearman]

HN does have a vote ring detector (or so I'm told) but I have a theory on how people avoid it:

You only need 4 or 5 quick votes to get to the front page, from there the submission will probably get organic traction.

So you need a network of 100 or more people who upvote each others submissions, but won't upvote if the submission already has enough votes to hit the front page. This way it's unlikely it will be the same subset of your network upvoting your story each time, making it much harder to catch.

Having said that, I have no such network, but have a few submissions reach the front page, one even spent a day at the top, so it is possible to get to the front page with out cheating