Something new to YouTube videos is a new visualization for related videos. This is something you can only see when viewing videos in full-screen mode.

In full screen mode, look to the bottom left, you’ll see a new three-ball button:

Click that button and circles will start to pop up on your screen. Hover over these circles and a speech bubble will come up telling you what the related videos are. Click on a circle and you’re taken to that video.

My opinion:

It would have been far better just for YouTube to show a grid (as in a larger grid compared to the one they show now in standard play) instead of cutesy circles that periodically move where you have to play "catch the circle" with your mouse. This seriously annoys the user (i.e. you).

Dopey feature, but it’s there.