As I understand it, it is similar to something my parents used when they were students. You can basically trace a picture's most important information onto a blank piece of paper by visually overlaying the image onto the piece of paper and tracing anything that you deem important. The advantage is that you don't have to approximate since you have the template right there.

Another way to think of this is if you printed the image and traced onto a piece of see-through material directly on top of the paper, only that you wouldn't have to incur the cost of printing, etc. with this method.