Kind of screws over tablet or smartphone users however.
(and no, google, i do not want to use your crappy mobile version, that is why i set my useragent to 'desktop', yet you don't listen!)
Actually I'm a little surprised there doesn't seem to be any keyboard shortcuts for invoking the preview.
Use the up and down keys to page through the results. When a result is highlighted, you can press the right key to show the preview.