One thing I have come across several times in the "Post your suspension mods" thread is the use of "Progressive Springs." I would just like some clarification as to what the progressive springs are atempting to accomplish.

In the suspension that I have designed, I have incorporated the progression/linearity of the wheel rate into suspension components and left the springs linear (engineering progressive/regressive springs aren't easy).

Basically, do the progressive springs you guys put in your cars bring the wheel rate to a linear rate, increase a fairly linear rate to a progressive rate, or change a regressive rate to a progressive rate?

Also, what are the typical applications for progressive springs (such as drag racing or autocross)?