As with many terms of its kind, the definitions are are many as they are vague - and creative!
Descriptions range from everything from a special kind of wire used for strangulation (sometimes by tying the wire in a noose around the victim's neck and then around their ankles) to pulling the tongue out through a slit in the throat, sometimes including the tongue being soaked in gasoline and lit on fire.
Sounds almost as painful as wearing a regular necktie.