Electronic clickies
Electronic clickies are an entirely different breed of clicky switch. They are not as heavily reliant on mechanical pieces to operate. Instead of physically bridging the gap between two electrical contacts like a normal mechanical clickie, pressing the button sends a lower power signal current to the control circuit, telling it to turn on. This results in less physical complexity, making such a clicky more reliable than your average mechanical clickies. One drawback to this design is that the control circuit requires a very small amount of power (compared to none at all for a mechanical clicky or even a twisty) to detect a "click", so if they are left alone for a long period of time (from several months to years, depending on the efficiency of the control circuit) they will drain the battery completely of its power. On many lights this power drain can be disabled by either unscrewing the switch from the flashlight body partly or by slipping a piece of non-conductive material (read: paper) between two metal contacts, usually the contact spring and battery. The most well-known electronic clicky light is the HDS/Novatac EDC, followed by the Photon Freedom and Ra Clicky. Many headlamps utilize similar technology as well.
Smart PD
The NiteCore Smart PD series of lights may utilize the PD design, but in their primary mode they use it differently than the original McGizmo design was intended. The Smart PD is actually a form of electronic clicky, and as with twisty PD's the major advantage is that they are a simplification of the mechanism and they put all the switching at the head which, theoretically should result in a large gain in reliability, although electronic clickies are not known for unreliability.
