The heretic's fork was a torture

Torture

Torture is the act of inflicting severe pain as a means of punishment, revenge, forcing information or a confession, or simply as an act of cruelty. Throughout history, torture has often been used as a method of political re-education, interrogation, punishment, and coercion...

 device, loosely consisting of a length of metal with two opposed bi-pronged "forks" as well as an attached belt or strap.

The device was placed between the breast bone and throat

Throat

In vertebrate anatomy, the throat is the anterior part of the neck, in front of the vertebral column. It consists of the pharynx and larynx...

 just under the chin and secured with a leather strap around the neck, while the victim was hung from the ceiling or otherwise suspended in a way so that they could not lie down. A person wearing it couldn't fall asleep. The moment their head dropped with fatigue, the prongs pierced their throat or chest, causing great pain. This very simple instrument created long periods of sleep deprivation. People were awake for days, which made confessions more likely.

Traditionally, the fork was engraved with the Latin word abiuro (meaning "I recant"), and was used by the various Inquisition

Inquisition

The Inquisition, Inquisitio Haereticae Pravitatis , was the "fight against heretics" by several institutions within the justice-system of the Roman Catholic Church. It started in the 12th century, with the introduction of torture in the persecution of heresy...

s.