From their about page:
<i>Tracer inserts a java script tag into the html code of your website to non-invasively track users interactions with your website content. The only change the user sees is when copied content is pasted into an email, blog or website, we automatically add a link back to the originating site at the end of the content.</i><p>What happens if the user just deletes the link, as I have with the above quote?
Years ago I applied a trick I learned from the Associated Press. Back in World War I, the Associated Press suspected that the Hearst newspaper chain was copying their stories from the Russian front, and they began running stories about a fictitious general Nelotsky. When the Hearst newspapers picked up the Nelotsky "story," the AP called them on it, pointing out that the first part of the general's name is just the English word "stolen" spelled in reverse. I used a similar technique on my most plagiarized webpage,<p><a href="http://learninfreedom.org/colleges_4_hmsc.html" rel="nofollow">http://learninfreedom.org/colleges_4_hmsc.html</a><p>mentioning a college that doesn't actually exist, but has a name from the Greek word for "steal." That finally got one persistent thief to acknowledge that my site was his source.
Great idea! Now we can finally, with accuracy, determine what portion of the internet population uses the click-drag technique when reading long blog posts. Yeah!