Piston Drive [or PD] in a nutshell is a proof of concept made by user Mcgizmo many moons ago. It was this design that propelled his reputation and lasted for many moons a custom light before it was "bought" by a company and made into a retail light.
the PD is basically a tube within a tube. The inner tube carries the battery while other outer tube acts as the body. rather than adding a push switch for levels, Mcgizmo allowed the battery tube room to slide up and down inside the body while still preserving the twist on/twist off function.
between the body and the battery tube theres two o rings, one near the bezel the other towards the end. Together they form a water-tight chamber keeping the battery and the electronics dry if the light is dunked in water.
I don't own a nitecore, but that being said, I love my McLuxIII-PD, sure its a U bin LuxeonIII, but who cares, theres no flashlight like it:thumbsup:
this thread:
FAQ: McLuxIII-PD, McLuxIII-T and McLuxIII-Ti-PD should tell you everything you need to know about the concept
Mcgizmo's product overview shows many examples of the PD in design
McGizmo's PD design is pure genius provides a little more on topic
The new nitecore design followed McGizmo's concept [with permission] and basically uses the same method in their lights.
Hopefully this should answer some [if not all] of your questions, let me know if it helps
cheers