The fork spring kit provided by Works Performance is actually two springs per fork leg. Unlike the progressively-wound springs sold by Progressive Suspension, the Works springs are two single-rate springs separated by a spacer. The spacer is threaded, with a screw running axially through the springs. The length of the screw adjusts the crossover point from the softer spring to the stiffer spring. When the forks compress enough, the screw bottoms out on the fork cap, and transfers all of the load to the stiffer spring.
(I know, I know, we need a picture here).