According to YouTube, the platform has been letting a growing percentage of its user base see these previews on their YouTube homepages, search results, watch page, subscriptions tab, and trending tab.
Since launch, YouTube TV has rather effectively used thumbnail previews to convey the live nature of on-demand channels and networks. This has been taken advantage of by content creators by using "clickbait" images for the thumbnails to entice viewers into clicking on their videos by having an exaggerated or even unrelated image in the thumbnail.
All you need to do is roll your cursor over the thumbnail in order to see 3 seconds of automatically selected video.
The lack of thumbnail previews made it hard to gauge the content of the video without clicking it, even though this technology has been available on some video sites for years now.
Some Twitter users appear to have noticed the moving thumbnails as far back as July 4, but YouTube officially announced them in a tweet it shared on July 13. The previews show up only on Chrome versions 32 and up and Opera version 19 and up; they don't work on mobile devices. If the feature does arrive for everyone - something that hasn't been promised, mind - it is indicated that creators themselves may get to choose which three seconds are transformed into the preview.
Are there any other features from alternate video sites you'd like to see YouTube incorporate? YouTube will itself pick up the random frames from the Videos and then preview it to the users who are looking for the video.