The operating principle is based on the fact that while a Dnyarri is manipulating a slave the Dnyarri, to some extent, shares the slave's sensations. Intense physical pain experienced by a slave can cause the Dnyarri enough stress and discomfort to cause it to involuntarily lose its psychic control.
After Kzer-Za announced this fact to the Ur-Quan population, Ur-Quan slaves began finding ways to break through their masters' control to inflict injuries upon themselves to weaken the Dnyarri's grip. As time went on the stubborn Ur-Quan slaves developed ever more and better means for doing so, until finally enough freed space and resources had been captured from the Dnyarri that Ur-Quan rebels were able to design and mass-produce devices that could be implanted in the brain, causing immense amounts of pain but no damage to the body. A well-calibrated Excruciator could keep the pain level just low enough for the determined, psychologically strong Ur-Quan to maintain a basic level of functioning while still making the task of maintaining control intensely difficult for a Dnyarri, particularly as the Dnyarri had at this point grown soft and complacent after generations of holding power.
With an increasing number of Ur-Quan shielded by the Excruciators, the overly stretched Dnyarri forces were unable to maintain their control and were quickly slaughtered. The bittersweet irony of using almost constant self-torture to defeat their enemies was one of the major factors that marred the collective Ur-Quan psyche after the war and led to the development of the Path of Now and Forever and Eternal Doctrine.