PD = Piston Drive
They are neither twisty nor clickies or both in one light, as you want it.
The light has a piston inside and on the board you have a positive contact and a kilroy (that's how Don calls a u-wire for contact). At the tail the piston looks like a Clicky, if you push it gently, the battery inside it makes contact and the light goes on low. If you push harder, the light will go on high and if you release the piston if will shut off again.
You can also turn the head like a twisty and the light will first come on Low, further on at high. If the light is on low, you can push the piston to get a momentary high.
The concept is ingenious, Nitecore used it later on for their EX10 and D10 with some electronic features, Don held it KISS. I have several of those lights now, the most unusual are the LS20/27, the LunaSols, which have 3/6 little LEDs (3mm?) around the reflector for Low and one big LED in the middle for High (all LEDs shine on High). Perform a search for them...
Be carefull, it always begins with one light, you'll want more and they are expensive. McGizmos are highly addictive and I fear it's already too late for you!!! :broke:
EDIT : As for upgrading, it is always possible to swap the LED and you'll find a few talented modders who'l do it even on a McGizmo. That's your choice, I'm personnally not (yet) a fan of it, at least not for custom lights. I have sent a light back to Don though with a problem, he'll fix it and I wouldn't mind an upgrade on that specific one. All my other McGizmos will stay as they are, my philosophy is to keep them as the artist designed them. As time goes by, newer and better LEDs appear however and if Don built the same lights today, he'd surely use a newer (better) emitter, a serious modder can do that for you and in terms of usability a good light will become even better.