"Voodoo dolls" are a common misconception about the religion of Voodoo and Vodou, they are something that was created and popularized by Hollywood. Voodoo is a religion, not a practice; although magick is enterwined with the religion, it forms no basis in it. Now "poppets" would be a better term to use, these dolls are used in many different practices such as in Hoodoo it is used to heal, protect, uncross, etc.
I literally just said it above, if you're thinking about putting needles into the doll expecting to harm a person that does not work; what you do to the doll like twisting his leg or arm won't do anything to the person, have to be pretty daft to believe it would. As I said before, these dolls are usually used for healing purposes, cleansings, uncrossings, protection, etc., the doll is used as a representation of the person but not in the same sense as what you see on the television