This trick is very clever: many sites limiting access to documents, let Googlebot (Google’s spidering agent) through. I think this is the case with some IEEE archives. So, you can simply tell your browser to identify yourself as the user agent “Googlebot/2.1”. Voilà! You can go where Google can go. What a beautiful hack!
With Firefox, you can get this result with the User Agent Switcher.
More details are available on Pinguy’s website.
Also see my post Google was eating all my bandwidth!