Inside every massage chair, you'll find a long set of rollers set on tracks that run down the entirety of the chair. If you were to sit in the massage chair, these tracks would run from the base of your neck all the way to the base of your lumbar spine. While some high-end chairs even have rollers that run down the underside of your hamstring muscles and even into the leg rest, massaging the back of your calves.
L-Track (sometimes called an SL-Track) works similarly to S-Track, although they extend further down the body. Where S-Track stops at the lower back, L-Track travels down through the glutes and all the way down the back of the lower legs.