Pain bands, also called training devices or pain-giving devices, were belt-like devices used by the Eymorgs of Sigma Draconis VI to control their Morg slaves through pain. The Morgs greatly feared this pain and quickly learned to obey the Eymorgs. Pain bands encircled the waist of the wearer, and consisted of a silver belt that connected a green module (at the abdomen) and a black module (at the lumbar back). Eymorgs operated the pain bands with their control bracelets.
While held in captivity by the Eymorgs, James T. Kirk, Leonard McCoy, and Montgomery Scott were tortured by pain bands several times. Drawing upon Spock's interface with the knowledge of the builders, Kirk discovered that a red button released the belts. Spock's body, at that time lacking its brain, was not subject to pain, so Kirk was able to operate it by remote control, causing it to seize the bracelet on Kara's wrist and then to then press the necessary button, releasing the bands.
The practice of using pain bands was probably abandoned when the Eymorgs were forced to the surface after Doctor Leonard McCoy removed Spock's brain from the computer control complex and restored it to his body, effectively rendering the underground Eymorg city uninhabitable. (TOS: "Spock's Brain")