A Judas cradle is a medieval torture device in which a victim is forced to sit on top of the pyramid-like seat with the sharp point inserted into their anus or vagina. Using weights and ropes, the victim would slowly be impaled by the device under the pressure.
After they capture Academy substitute teacher Rika Shiraki, Reika Kitami's evil followers Kaori Saeki and Saki Kiriya use the device to brutally torture her as punishment for supposedly spying on them. They tie weights to her body and inflict horrible suffering on the woman's body.
They later threaten to do the same to their former ally Yuki Toudou when the young student reveals herself to be a spy for the elusive Jody Crowley. A fitting punishment for betrayal, considering the device's reference to JudasWP.
Shiraki is eventually released from the device, and proceeds to urinate on the floor before her next round of water torture starts.
Shiraki is strapped to the device.
- Judas cradle on Wikipedia.