I'm finding this part of the chapter quite interesting. When I googled Premack Principle I came up with a number of articles/websites mostly about training your dog. Then I found this one... "STOP Prograstinating! Use the Premack Principle." Ok I said time to do my homework, but first let me read this article.
After reading the paragraph in the book and this article I can sum up his principle easily by saying, do your homework first before updating your Facebook status.
This is an excellent method for those students that might be more challenging in your classroom. By giving them choices and puting them in a order of completion students can be successful.
Check out this website: http://gettingstronger.org/2010/12/stop-procrastinating-use-the-premack-principle/