In psychology 101 (but not found in cardiology 101), budding psychologists learn the difference between independent and dependent variables. An independent variable is the event you manipulate (e.g. a whack on the head), and the dependent variable is the response (e.g. someone whacks you back) that correlates with the independent event. For the relaxation response, relaxation correlates with focusing on something while sitting in a distraction free environment. Funny thing though, Benson never thought that sitting in a distraction free environment is an independent variable as well (this is called resting). But that's just as well, as sitting by yourself away from mental or physical distractions is a generic hypothesis that would likely fail in book or therapeutic format. Benson could have resolved the issue by showing that paying attention to a simple stimulus (e.g. driving a car in a rain storm while trying to focus on the little white stripe in the road) would cause relaxation no matter the level of distraction, but obviously felt that the relaxation response was so true, it would be insulting to test it further. As for the dependent measure of relaxation, relaxation has always meant (well at least prior to Benson's genius revelation) that the muscles are for the most part not doing anything, or are relaxed. Thus the relaxation response, or a 'doing nothing' response, is in fact an oxymoron, but nonetheless is accepted by morons the world over.
Go here for more of my sarcasm on the relaxation response.