The Light Spring replaces the Standard (Heavy) Spring which comes in the Fusion Engine and F2 HPA systems. The Light Spring is used to help equal the force balance to improve feeding and function when running the system in the lowest pressure range. This is also the "standard" spring that comes installed in the Kythera.
The Heavy Spring is what comes standard in Fusion Engine and F2 HPA systems. It is also the "extra" spring that comes with the Kythera system.