please someone explain a major piece of this movie to me! *spoiler*

Rating: Null/10 (23 August 2003)

Hi, My boyfriend and I just finished seeing "Pirates of the Caribbean," and afterward we both realized we had no idea why the cursed pirates of the Black Pearl needed the blood of either young William Turner or his father in order to lift the curse. We have many speculations, but none of them quite seem to add up, such as... Perhaps because Bootstraps, as one of the cursed pirates, gave one of the coins away to his son, so when the coin is returned his blood must accompany it, but he's at the bottom of the ocean somewhere, so his son's blood must take its place. However, that would mean that every pirate would need to shed his own blood every time a coin was returned that the particular pirate spent, and mostly the pirates are just skeletons (although they do seem to occasionally bleed)? Basically, we think we must have missed some major step in the movie, but I spoke with my sister who's seen "Pirates" twice, and she seemed just as baffled. Someone please enlighten me, and fast! Thanks!