When you click a link, your browser sends the URL of the site where you clicked the link to the server that hosts the link you're clicking. This is called 'Referer' (sic)[0] and can be disabled in your browser [1].
Now, the owner of that blog set it up in a way that people coming fon HN (who send a Referer) are redirected. (Another user figured that out a few minutes ago: https://news.ycombinator.com/item?id=26078619 )