Jack took a coin out of the chest and kept it, so that he could not be killed. After all, he was going up against four pirates that couldn't be killed in order to save Will. He knew that the odds were better with the rest of the pirates gone, but still it was a long shot for Jack and Will to win.
So when Barboosa runs Jack threw with his sword, he falls back into the moonlight, where he becomes a skeleton, just like the pirate crew. Because he took a coin from the chest.
Therefore, he had to put his blood on the coin and return the coin to the chest to rid himself of the curse.
Savy?
Born same place as Johnny Depp |
