You can Doodle on a video just as you would Doodle on a static image or document. When you are commenting on a video, however, you have the added ability to navigate through the video as you comment. This allows you to navigate directly to a specific point of the video. The frame that you see on your screen when you record is what others will see when they view your comment.
See the Doodling tutorial video for a step-by-step illustration.
Video Doodling in action!