MX Progressive springs follow an exponential force curve. They start light and get exponentially heavier as you approach bottom out. They are highly recommend for those learning to touch type. They are within 1% deviation.
There is no polarity on progressive springs. As SPRiT says:
"According to the Newtonians, force of object A on B = the opposite force of object B on A. It damn doesn't matter which end. You feel the pressure exactly by the pressure you press"
Further descriptions can be found on SPRiT's website.