According to Shuar beliefs, there are different kinds of souls. The arutam soul is one that, when it's in a body, prevents complete death. When a person die, it calls into existence an immortal creation called the arutam wakani, which is heralded by storms or heavy winds. A person who possesses an arutam soul can create something called the muisak, which is an avenging entity that can take the form of a demon in order to exact revenge for the death of the body.

That's where the practice of creating the shrunken heads comes in. The muisak lives in the person's head, and if the corpse is left alone, it gives the muisak the freedom to travel to any other dead body that it needs in its quest for retribution. Ultimately, it will take a natural form, favoring animals like poisonous snakes. Taking the head of a dead person and shrinking it is thought to trap the muisak inside and prevent it from getting its revenge.

If someone has done wrong against you, all you need to do is inform the muisak at night when he opens his eyes. Whisper into his ear the name of the person who has done you wrong. This will send curses and spells against that person; tenfold what they have done to you. The muisak must be completely subservient to his owner and grant all of his requests. You can ask the muisak for anything you want and your request will be manifested in your life regardless of what you have asked for.