While voodoo has a lot of roots in religious and spiritual practices of the human race and is very extensive, for cogs, voodoo specifically refers to healing and charming/hexing objects. A witch doctor cog can do so with a supernatural magic they possess that's incredibly innate. A cog cannot become a witch doctor without having at least a small amount of natural magic, but those who possess a lot of intrinsic magic will fare better.
In order to become a witch doctor, they have to be annointed as one by a mystic force from their chaos realm, Fiafuzion. If they're carelessly given these abilities by anybody else, havoc will arise all over the land. (You'd think that a chaos realm would love the sound of that, but it's actually quite contrary.) A bot must learn how to channel their personal magic into an object once they've become one. The object doesn't matter, as long as it can be lodged into their crystalline heart it will work, because that's where it must go.
They give off grandiosely unsettling and occult vibes, but those are caused by hexed piecings they must adorn themselves with. Each witch doctor operates differently and heals/enchants objects in their own personal way with their own personal motives.