It's the last statement in the processSearchResults function:

jQuery("a.short").mouseover(function(e) {
  jQuery.get("?",
    {
      // convert preview.tinyurl.com to just tinyurl.com
      expand: e.target.href.replace(/preview\.tinyurl\.com/, "tinyurl.com")
    },
    function(data) {
      jQuery(e.target).html(data).unbind("mouseover")
    }
  );
});