[alttag]

  > ... clicking an up arrow on Hacker News submits a GET request

For the curious, (from peeking at the source) it's a anchor tag with onclick Javascript that overrides default behavior if js is active. Before checking, I thought it might be AJAX, but it's much simpler. The script simply loads a background image with the info in the link (but doesn't add it to the DOM, so there's no reference after the function returns.

The entire relevant server communication portion:

  // ping server
  var ping = new Image();
  ping.src = node.href; // Where node is a ref to the clicked anchor