To use Google Instant Previews on your mobile device, all you have to do is make a search on google.com and tap on the magnifying glass next to any search result. A side-by-side comparison view of the webpage previews for the first page of search results will appear. When you find a result you like, tap on the preview to go straight to the website. When there is a mobile version of a website available, Google makes a point to show a preview of the mobile page.
Google has created a video to promote the feature:
Google first launched Instant Previews for the desktop back in November 2010. While I see how it could be useful, I have personally never bothered using the feature. Have you given it a shot, and if so, what was your experience?