The NiteCore IFD2 is the 2 AA light in the Infilux line, adding another light in addition to the CR123 IFE1. The Infilux lights have a regular switch on the back to turn the light on and off, but there is a rotary ring on the head as well that will give you adjustable brightness, from less than one lumen to 260 on the high end. One complaint about the IFE1 is that it was hard to get and keep in the lowest brightness since the lowest brightness was so close to the detent before going into strobe. The IFD2 solves this by giving you a bit of extra room before it goes into the lowest brightness. The lowest brightness is rated at less than a lumen, but there are actually two brightnesses even lower than that. I would estimate that the lowest is more like .01 lumens, which is completely and utterly useless for illumination purposes, but it sure is fun to mess around with and see.
The IFD2 is quite a bit longer than the IFE1, but is still slim enough for reasonable pocket or pack carry.
See it in action below. I mess around with the lowest brightness as well, just to show you what it can do.