This energy heats up the collagen and the layer of skin that produces collagen. One characteristic of collagen is that when heated, it tightens; hence skin appears firmer and younger. And when the layer that produces collagen gets heated repeatedly over time, it will be stimulated to produce more collagen.
At higher energies, radiofrequency can be used to heat up the fat layer and to shrink fat cells. This application is useful for treating cellulite and sculpting of the face and body.