Most simply put, a servo sub has a mechanism (the servo) that senses when the driver is getting out of control... maybe it should be pushed right up to the limit, but it goes past it instead (maybe even bottoms out). The servo mechanism brings this back under control and back in line with the incoming signal, thus reducing distortion... because distortion is, by definition, any deviation from the incoming signal.
As has been said, servo is a feedback device designed to reduce distortion. Some incarnations are better than others. Velodyne is probably the most noted servo sub.