Thio relaxers use ATG, or ammonium thioglycolate, which is also used in permanent waving, but at a much higher pH and concentration than used in permanent waves. It is usually higher than a pH of 10. These relaxers are also thicker with a higher viscosity, or thickness, which makes for an easier relaxer application.
Thio relaxers break the disulfide bonds in hair, similar to the permanent waving process. When enough of the disulfide bonds in the hair are broken, the relaxer is rinsed from the hair and the hair is towel dried. After towel drying, a neutralizer of some sort is applied to the hair.