accused of overseeing the torture and execution of around 15,000 people held at Tuol Sleng prison in the late 1970s.
"We were taught how to torture the prisoners and to avoid the prisoners dying, otherwise the confession would be broken and we would be punished," Prak Khan told the court.
The witness, who became an interrogator in late 1976 after initially being assigned to work as a prison guard, said Duch and other high-ranking Khmer Rouge cadres often taught torture methods.
"We were trained how to whip the prisoners with the sticks, on how to electrocute, (and) how to use the plastic bag to suffocate them," Prak Khan said.
He said the court interrogators were also taught a "light but painful" torture method in which they inserted a needle under prisoners' nails, and that sometimes inmates were forced to eat excrement.
"Detainees would be told not to make loud noises, not to curse or exchange swear words, or to shout slogans. And they were also warned not to scream while being tortured," he said.
He said also he occasionally witnessed medics extracting blood from prisoners until they died.
"As I noted, there were five bags of blood taken from one detainee until the person was dying. After blood was drawn, no one could ever live because they were dying already while their blood was being taken," said Prak Khan.
Prak Khan said interrogators would torture prisoners until they confessed to spying on the Khmer Rouge regime and provided names of others in so-called espionage networks.
In earlier testimony, Duch admitted he did not believe that confessions obtained through torture were accurate.
Prak Khan recalled that children were separated from adult prisoners and exterminated, adding that he once saw a Tuol Sleng guard kill a seven or eight-month old infant.
"(A guard) took the baby from the mother and he dropped the baby from the upper floor to the ground and later on I was ordered to bury that dead baby," Prak Khan said.
The 66-year-old Duch, whose real name is Kaing Guek Eav, has accepted responsibility for his role governing the jail and begged forgiveness near the start of his trial for war crimes and crimes against humanity.
But the defendant has consistently rejected claims by prosecutors that he held a central leadership role in the Khmer Rouge, and says he never personally killed anyone.
Led by Pol Pot, who died in 1998, the Khmer Rouge emptied Cambodia's cities in a bid to forge a communist utopia. Up to two million people died of starvation, overwork and torture or were executed during the 1975-1979 regime.
Four other former Khmer Rouge leaders are in detention and are expected to face trial next year at the court, which was formed in 2006 after nearly a decade of wrangling between the UN and the Cambodian government.
The troubled tribunal faces accusations of interference by the Cambodian government and claims that local staff were forced to pay kickbacks for their jobs.