Waterboarding

A criminal investigation interrogation technique whereby a person suspected of having or withholding relevant information is blindfolded and bound on their back, sometimes with the fact covered with porous or nonporous material, and subjected to water poured over their mouth and nose such as to simulate drowning and to thus, under duress, elicit information.

The interrogation technique leaves no permanent physical sequelae.

Many international agencies and experts debate whether waterboarding is torture. 

As far as admissibility of evidence is concerned, the issue ought to be moot as any information obtained under duress leaves little place for truth and, generally, encourages a dangerous relapse to an era of law enforcement which uses torture or inhuman, cruel or degrading treatment or punishment.