Yes, Adaptive Luma.
Gamma is not the simplest concept, but is not too hard to understand. It has absolutely nothing to do with color directly; adjusting gamma will make any particular color darker or lighter, but will not adjust the chroma level or color phase in any way per se, although they could theoretically become unbalanced due to a gamma shift, just not likely in practice.
It only adjusts the luminance, and only can affect color due to that. It also adjusts luminance non-linearly. Brightness and contrast adjust luminance linearly. Turn up brightness and everything gets raised by a linear amount.
The only way I can describe it is if you first envision the scale of luma to be steps of gray on a graduated scale between black and white. A gamma shift will adjust all levels of gray, but non-linearly, meaning it will adjust some of the different steps on the scale up or down more than it will others. The closer to black or to white, the less adjustment, while the closer to the middle of the scale or 50% gray, the most adjustment. This means that a true gamma adjustment by itself will not adjust pure black or pure white at all, so will not mess with your preset black point and white point.
But the "Extend" setting adjusts gamma while also messing with white and black, which is why you need to set it first, and then set your brightness and contrast and backlight. It may give you the impression that the picture is shifted darker or lighter, but that is only because part of the picture actually is brighter or darker, just not the parts that are closest to 0 IRE or pure black and 100 IRE or 100% white.
True gamma allows you to give the impression of overall brightness change without moving the black and white points at all, which can make the picture more watchable without really making it brighter, and in the case of "Extend", can stretch the lower part of the gray scale to allow detail to pop out even if it is close to black or in the "pluge" section of color bars (the HDNet test pattern can be very helpful setting this using your brightness, contrast, and backlight), but all of this works best if you click "Extend" on first.
If you have a Mac, experiment with the gamma graph settings in the "Displays" system preference panel. Windows has something on the same order, but not as advanced. You can also play with these graphs in Photoshop or Elements from CS suites if you have those installed. That will give you a more intuitive understanding of gamma.
If the scale of brightness is graphed as a straight line on an X-Y axis it will be a straight diagonal line from left-bottom to top-right. Adjust gamma one direction or the other and the ends of the lines (representing pure black and pure white) never change, but the middle of the graphed line will either balloon up or sink down, turning into a curve, which is where we get the term "gamma response curve". Extend adjusts gamma to balloon the curve up, meaning what was once 50% gray will be more like 65 or 70% gray, and what once represented 0% to 5% gray now may represent 0 to 10% gray, which gives it the headroom to allow dark detail to resurface and pop out.
"Extend" also moves the ends of the graph to some extent (typically brightness will move both ends up or down in unison [everything gets brighter or darker linearly], while contrast will move one end up and the other down or vice versa in unison [whites get brighter while blacks get darker or the opposite], while the graphed line stays straight). "Extend" probably raises the low end and more than the high end, which is why it is not a pure gamma adjust but a combination of gamma and brightness and/or contrast.