The victim that sat in the chair had all sorts of gadgetry, piping and wires wrapped around them to increase their terror. The chair was able to make a victim's muscles to jitter and spasm. The seizures that were caused were not painful so much as they were maddening. A most effective interrogation and torture technique which this machine exploited was to create an unrelenting facial tics that would cause the victim to blink over and over for hours on end.
When Han Solo and his companions were captured by Admiral Daala and her fleet, this is the device that Daala used to interrogate him. Daala then proceeded to ask Solo some questions about what had happened to the Empire and he refused to answer. When she used it to make his left thigh and his right hand spasm, he then told the whole story of the Galactic Civil War to her.