The D property determines how much dampening will be applied to the force used toward reaching the goal BodyPostion/Position|Position. When the part approaches the goal position it needs to decelerate, otherwise it will move past the goal and have to stop and re-accelerate back toward the goal. This is often creates undesirable rubber-banding effect, so applying dampening using this property is how that effect is avoided. The higher this value is set, the greater the dampening curve becomes, or the slower the part will approach the goal position.