They aim at increasing the temperature deep in the skin up to the critical limit of 43°C. In this temperature a significant influence starts on the protein bonds that hold the collagen molecules stable. This way a denaturation of the collagen molecules takes place, which leads in its shrinkage. This shrinkage is interpreted by the body as injury and so the skin is ordered to produce new collagen to repair the damage. This way quantities of new , stronger collages are deposited on the skin, in a rate that the skin had at younger ages. 

At the same time the temperature causes  vasodilation  resulting in local hyperemia ( increase of blood flow ). The improvement o f circulation affects fat cells as well since their metabolism is increased and so they return to their normal dimensions. This way the skin loses the appearance of orange peel and the body contour is sculpted and improved.