It refers to the establishment of the coils...
Regular springs have evenly spaced coils throughout the entire spring

Irregular springs have variable coils. Usually around the two ends are evenly spaced BUT the middle is either spaced further away or closer together.
**It's not a spring, but look at the concept**

Non-linear springs don't really have a pattern, they're typically tightly coiled on one side, get a bit looser, then can tighten up again but not as tight as the first end.

I've yet to know how true this is, but apparently, the idea is non-linear and irregular spring compress better and easier in comparison to regular springs. Apparently this can be better seen/noticed with heavier springs.

 

Yes, Buster is right. The way irregular pitch springs work, is similar to the principle of a bungee rope, but in fact is the opposite. The looser coils compress more easily giving therefore giving the tighter coils more inertia, keeping it in motion for longer, thus stressing the motor less.

 

So a irregular spring would be the best buy ? Trying to figure out a good a spring for my dmr