Progressive rate springs are designed to ride better at lower speeds. The progressive rate springs I've seen have the softer coils farther apart at the bottom of the spring, with the coils getting closer together toward the top of the spring as the rates stiffen. You get a softer ride over bumps at slow speed. but as you start driving harder, the springs stiffen up.
It is a good compromise for street use, although most pure track set ups use single rate springs so that handling balance is not upset by differential stiffening between front and rear springs.
Jim
