10th century viking torture tactic in which the victim is tied up to a pole or wall with arms spread eagle style and back side out. The torturer then makes two cuts on both sides of the spine (one long slice on each side) to open and expose the victim's lungs. Generally, because they are more commonly tied to a pole, the lungs naturally pop out of the back. It is also common that, out of pain, the victim writhes and struggles to escape from being tied down and causes the lungs to flap about on their back, thereby looking like a "butterfly."
The police found the child: he was dead hanging on a cross butterflied by the kidnapper. He'll be pinned for murder now, especially because of the butterfly cut.