Toggling between full screen and video page modes was cumbersome on Laptop. So YouTube has introduced a 'Scroll for details' feature which lets you scroll down and read comments and other information while staying in 'Full-screen mode'.
Why it's Amazing?
1. This is a very small feature for YouTube but it proved the quote "Great things are done by a series of small things brought together."
2. The design of this feature creates the good user experience because it doesn't change it's behavior while scrolling, the Fullscreen remains as it is.
3. Also, while scrolling, User can listen the audio, search for other videos and navigate to other menus.