How they work is through perfectly natural phenomena that have nothing to do with the “supernatural”.
Unlike the press of western anthropologists, the vast majority of people practicing African indigenous spirituality, whether traditional Vodun and Orisha or Bokoko and Kindoki-Brujeria, are not literalists. Meaning, we do not believe there is a dude named Eshu literally sitting at the crossroads or in every corner or border. The world would be pretty crowded then, wouldn’t it?
What we believe is more that there is a principle of liminal space. How this is arranged or was arranged at the beginning of the Universe or material reality is a mystery to us, but we can use this principle for practical purpose. We can use any and all means to defend ourselves and mind our personal boundaries, invoking the idea of Eshu to aid us in this, and fill in whatever blanks in the subconscious that the mind can’t consciously grasp.
We can make an amulet that will remind us of this principle, and through its construction, ingredients, and maintenance as a “fetish”, boost this impulse in ourselves as well as signal to others through subtle psychological, visual, verbal cues and whatnot, that we are the wrong target.
We can worship this principle as an essential aspect of ourselves, the divinity in others, and the world around us. We can be grateful for our very sanity so we don’t regularly marry flowers instead of humans and therefore fail to reproduce *unless* that is someone’s call, then someone can marry a river to protect it.
So much of “Voodoo” is framed by westerners as either “magic” or “supernatural” in their limited terms. For us it’s just super natural.
Why or how it works is secondary to the fact that it works. We are grateful that Nature works, and this gratitude has worked well for us whenever it is embraced and acted out through observance and workings.