In progressive springs each coil is spaced differently and have a variable spring rate. When free, it is easy to compress progressive springs for first centimeters. As you apply more forces, coil on a progressive spring come closer. After a certain point, coil at the top 1/4 of progressive springs begin to touch each other and finally become inactive or dead, and that makes the spring stiffer. Apply more forces to a progressive spring then it becomes stiffer because as the number of active coils in a spring decreases, the spring rate increases. So, progressive springs may both be sensitive to very small bumps on the road, while giving the stiffness you need during hard braking and turning.
