It's not actually loaded, it's prerendered. The rough details are here: http://blog.chromium.org/2011/06/prerendering-in-chrome.html
The salient bit seems to be: "The browser fetches all of the sub-resources and does all of the work necessary to display the page. In many cases, the site simply seems to load instantly when the user clicks."
I'm not sure if impression tracking gifs would be loaded (I'd hope not), but it doesn't seem to run any of the JS.