Back in 2017 we saw Google testing autoplaying video previews in the knowledge panel. Now it seems Google is autoplaying video thumbnails on overlay in the video carousel box in the search results.
I personally can replicate it for a search on [how to play the blues], as Jason Barnard shared on Twitter. Here is my GIF of it in action:
You move your mouse cursor over the video box and it previews a snippet of the video directly in the search results.
Frank Sandtmann also sent this to me the other day and I am able to replicate this, so it might be fully live on some more modern browsers.
Forum discussion at Twitter.