Basically, they've taken a snapshot of each page in the Google index. Then users just have to rollover a magnifying glass icon, so the destination page is previewed before a user has to click through.
The keyword search does not return faster, per se, but the entire experience is built to make the decision-making faster -- thus, hopefully, more satisfying.
Google 'splains: "After clicking once and opening a preview, you can hover over other search results to preview those pages. For some previews, highly relevant parts of the page will appear in text call out boxes over the preview image to help you see where your query appears on the page."
There's a Labs site up and running. Full roll-out comes soon.
We don't have it running at our content warehouse in San Jose, but our breath is bated.