Hidan's voodoo technique is in fact not a jutsu at all. It isn't even "real" voodoo. I believe there is a trick to it. A trick that explains how it works and that it really isn't what it seems. The transformation Hidan undergoes is purely for decorative purposes. The blood licking is an act of symbolism. Drawing his opponents blood means that he is worthy of winning the fight and that he is worthy of power. What the power is is the service of a summon. Hidan's summoning technique is some kind of invisible monster. The monster requires payment for its services and the currency is suffering. He will do unto others as Hidan does unto himself, so to speak. When Hidan suffers, so will the victim that he is linked to.