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Water boarding has been around for centuries. It was a common interrogation technique during the Italian Inquisition of the 1500s and was used perhaps most famously in Cambodian prisons during the reign of the Khmer Rouge regime during the 1970s. As late as November 2005, water boarding was on the CIA's list of approved "enhanced interrogation techniques" intended for use against high-value terror suspects. And according to memos released by the U.S. Department of Justice in April 2009, water boarding was among 10 torture techniques authorized for the interrogation of an al-Qaida operative. In a nutshell, water boarding makes a person feel like he is drowning.

Water boarding as it is currently described involves strapping a person to an inclined board, with his feet raised and his head lowered. The interrogators bind the person's arms and legs so he can't move at all, and they cover his face. In some descriptions, the person is gagged, and some sort of cloth covers his nose and mouth; in others, his face is wrapped in cellophane. The interrogator then repeatedly pours water onto the person's face. Depending on the exact setup, the water may or may not actually get into the person's mouth and nose; but the physical experience of being underneath a wave of water seems to be secondary to the psychological experience. The person's mind believes he is drowning, and his gag reflex kicks in as if he were choking on all that water falling on his face.

http://science.howstuffworks.com/water-boarding.htm