First, I would like to state that after this lesson, I have this Coherence Principle down! Not only have I read (and re-read) the chapter in the Clark and Mayer book but I have researched this principle all over the internet and spent much more time than I expected on answering the questions associated with this assignment.
In a nutshell the Coherence Principle states that in order to maximize learning we need make sure that all of the media in the lesson is essential to the learning process. Any extraneous media (sounds, graphics and even text) should be eliminated from the lesson. Often times teachers add in other graphics or movements to “enhance the content” or to “keep the students engaged” or “make it fun”, but studies by Clark and Mayer show that this actually detracts from the learning outcomes.
You can read all of my responses in my Google Doc on the Coherence Principle.