Remember the old feature in old Netscape browsers that prefetched links on a page so that when you clicked on it, the page would load from your cache instantly? Google's Instant Pages seems like that.
Here's how it works: You do a Google search. When you click a search result (usually the first one, because Google guesses you want the most relevant link), the resulting page loads almost instantly. So Google takes the time you use to choose a search result and uses that to prefetch a result into your browser's cache, so when you do click it, you'll load it really fast (from local storage).
Google says it only does this when they're very confident that you'll click the result, and then downloads and "pre-renders" it in Google Chrome, to save even more time.
It's very, very fast when it's in action, and Google says Instant Pages saves 2-5 seconds for every search. This is on top of the Google Instant feature, which already saved you about 2-5 seconds.
Instant Pages will be available this week in Chrome Beta, but today in Chrome's developer version. They're also working on Instant Pages for mobile in the near future.