Can you explain why thefollowing device doesn't act as a perpetual motion machine?

it's basically just a pendulum with two magnets. The magnets are placed so they attract the pendulum bob at the top o fits swing; however there is also some kind of shielding device which swings into place (and is also powered by the pendulum) as the pendulum bob reaches the top and thus cuts off the magnetic attraction. A link to an article explaining the fallacy would be ideal; it's a common idea that many people have had.