DED, I found this info on another website
The interactivity is created by overlaying annotations on top of the video. Annotations were previously used solely as captions or speech balloons leading viewer to another site for more information related to the video. Now they are transforming YouTube from a static video player to a tool for engaging the audience. Check out YouTube's help section for more information on how to create annotations.
Zolhof and his band made fantastic use of it too.