They dont. Or, i suppose you could say 'by self-fullfilling prophesy of the superstitious fool'. For this method you dont even need an actual doll tho. Just tell the one who believes such things that you stabbed a doll of them and theyll start having issues. Only wirks on fools who believe in 'voodoo' tho.
Technically speaking, they don't. But if the intended victim believes they work, and knows or suspects they are being used, then the victim psychologically creates his or her own problems as a result. Believing in the supernatural can be dangerous -- as is living in ignorance.
I understand you had an idea before acquiring that voodoo doll! yes many people can discourage you but if you really want to do what you want you can make it to work, concentrate on what you want to achieve using that doll as a get way to reach to your target. It can require some other ingredients to reinforce your actions depending to your target. http://afrohealer.co.za/2012/07/binding-love-spells-that-work-fast/