Symbolism, Imagery, Allegory
It seems like Toussaint and Shorty are a little skeptical about pwens at first. It's a bunch of voodoo to them, and when we first hear about the pwen, we're told:
In it was one of the gods the Dahomey had brought with them to this new world, a god of war called Ogou Ferro. Or so Boukman believed. After kissing the stone, he slipped it back into his pocket. Boukman believed the stone would protect him from those who wished to hurt him and would help him in times of need. Toussaint thought it was just a stone. (2.56)
Um, okay. Before long, the pwen becomes more important to both Toussaint and Shorty. They both come to believe that the pwen helps protect them. Now we're not saying that's right or wrong, but it's important because of what they believe. Once they are kept safe time and again (when they really should be making funeral arrangements), they start to buy into the whole magical power thing.
In fact, Toussaint gives his pwen to Isaac to protect his son by the end. Not only does the pwen symbolize magical voodoo powers to the characters, it also represents the shift in thinking that our two guys undergo in the novel.
That just leaves us with one, burning question: Where can we get a magical rock to make us bulletproof?