Crunches with additional weight -- abs or arms?

Hi Scooby and everyone else -- question about your ab workout.  In your ab workout video, you demonstrate crunches done with additional weight behind the head (first the max you can do, then 50% of your max).  My questions is, how does this weight benefit the abs, since (despite the fact that it's being held behind the head/neck) it's actually being lifted by the arms?  Is it because the weight is actually on the shoulders, and in a crunch it's the shoulders that are being lifted off the floor?  

Thanks for the website and all the advice and videos!