I found this in my research:

What makes a spring progressive is a geometry where some of the coils
bind up partway through the anticipated travel. For instance let's take
our original spring and play with the coil spacing. Let's wind it so
that the first two coils only have 1/4" of air between them while the
other two coils have substantially more. Put 100# on the spring, it
compresses 1". But at that point, two of the coils can't compress any
more. Add another 100# and it only compresses 1/2"; i.e. the rate of
the spring has doubled to 200 lb/in!

Although a change in coil (or wire) diameter does not make a progressive
spring, it can be used to make a spring more progressive. If we take
our second spring and make the two smaller diameter coils widely spaced
while the bigger coils are only 1/4" apart... Put 100# of force on this
spring. It compresses 3/4", rate is 133 lb/in. Add another 100 lb. The
big coils are already bound up, so only the small coils can compress;
they each compress only 1/8" for a total of 1/4" or a 400 lb/in rate.
So instead of a 2:1 change in rate, we've now got a 3:1 change...


Peace, Jon