The inner piston spring controls the return action of the inner piston. Like all springs it will get weaker/softer with time. If the inner piston starts to clatter (rattling sound) it can be a sign that it is too weak. To resolve this you can either replace it with a new or you can stretch it about 2mm. Also make sure that the inner piston is oiled and moves easy.
This spring can also be used with the Stingray when used as a shader.