Originally Posted by
billetbirdcage
I wouldn't say it would defy the laws of physics but would be very unusual metal or way of doing it to make a single spring digressive. Think of it this way: A typically balloon. When empty the rubber has more spring rate and once expanded it gets softer, basically easier to expand it with less force the more it gets filled. Most people are going to think some sort of metal but if you start thinking about other materials, I think many will see it's possible.
If the material was stiffer in it's free state and became less rigid/stiff as the metal started to bend/bow, you could make a digressive single spring. Not sure that metal exists or they are using a different method to get the same effect. But it seems possible to me as there is some pretty amazing stuff out there in alloy's and composite blends. It would likely be way easier to do with some sort of composite material as a spring doesn't have to be metal.