If you check Inspect Element when hitting Send Feedback, I believe you can see s...

ebzlo on July 27, 2011 | parent | context | favorite | on: Html2canvas - Webpages Screenshots with JavaScript

If you check Inspect Element when hitting Send Feedback, I believe you can see some of their magic (includes HTML5's canvas element and flashcanvas.swf) which leads me to believe it's not done server side.

Also modifying the DOM preserves when hitting Send Feedback.