Google has added the Instant Previews for videos option to its video search results. It is an enhancement to the pre-existing Instant Previews feature. With it, now you can preview videos instead of watching the full clip of all the videos returned for your video search. This will speed up your video search and allow you to get to your video faster and watch more.
When you run a video search, you can see a clickable magnifying glass on the right side, next to title of the video. Now, instead of viewing a preview page of the website, you will see playable clips from a video posted at the site. You can let the preview videos play by themselves or hover the mouse pointer over the one you want to preview. The preview clips has no sound by default. Therefore, in case if you want to play the sound as well, you will have to turn it on by clicking the little blue 'x' symbol next to the volume icon of the preview clip. If you want to open or go to the full video, you can click the preview clip.
Since this option has just been added, not all the videos have this preview option yet. Hence, you can only see them when video providers have made the previews available for their uploaded clips.