Example search results for 'apple' on DDG: http://duckduckgo.com/apple
If you scroll down a couple of times more results are fetched with ajax. Then you click a link, visit the result, click the back button, and the DDG page is exactly as you left it.
How?
I noticed that the d.js file is appended to on the page and it contains all of the result links. Could this be how they do it?
If all links are loaded from the JS file and your currently cached version of the JS is the last (largest) page of links, that would allow all results to remain. Not sure how they are forcing cache of the d.js file since it contains a query string though.