To understand the power of the voodoo doll, we first have to get into a bit of voodoo beliefs. There are many rituals in the voodoo faith, and any time items are used in those rituals to link to the spirit realm, those items become "pwen," according to Learn Religions. The purpose of a pwen object is to appeal to a deity of the voodoo faith, entities referred to as Iwa or Ioa. Each pwen is specially made for a particular Iwa, and would include objects that spirit is drawn to. The end goal was to recruit the support of that particular deity in whatever the ritual is meant to achieve — perhaps safe travel, good weather, or better focus. 

Not every pwen is a doll, but dolls are one type of pwen, and one of the more common ones. Not all pwen are even physical objects. They can be practically anything. For the doll versions, however, Learn Religions points out that the doll can be anything from an elaborate effigy to the simplest of representations. But their purpose is the same: to heal, to protect, and to guide. 

Once the object has served its purpose, the object loses its pwen, and the spirit returns to wherever it came from, leaving the effigy as little more than a statue. 

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