I was looking at the YHM link. I'm guessing that since it's gas piston, you can adj the pressure that cycles the bolt.
If you can adj the pressure cycling the bolt, you can reduce it.
If you can reduce the pressure, you can use lighter recoil spings.
Since they are using a folding stock, that would mean that the bolt carrier is abbrivated (shorter) If it were a standard bolt carrier, it wouldn't cycle without the buffer tube.
With a shorter bolt carrier, the lighter buffer spring can go into the upper receiver and close down on to the lower.
Of course I don't know 100%, but this would be my best guess.
Hardwarz