Thechroma key technique is just based on the chroma difference betweem an image and a reference, this permit to isolate any color, mine is just for red green or blue screen. But mine give softer edge and is a much prefered method by many compositor for greenscren or bluescreen.
A chromakey is an historical method form the analogous video world (where signal is YUV) so it is just "a switch" driven by the UV value of the image. Just like a lumakey does with Y channel.
Furthermore, you will need to despill, color correct and composite (with edge blending blur better) this will need a lots of nodes (arround 50) with a lot of redondant/unsused parameters.
My node does all that internally and permit you to choose different method for some operation (no more confusion with mixing the noodles conexions).
If you can put your hands on "Digital compositing for film and video" by Steve Wright, I deeply suggest it to you, it explains all the keying techniques and much more:
Also, if you don't use my node, you can get better result using the math node (lots of them) then using the chromakey, beleive me chromakey is not the best way to do greenscreen or bluescreen, but it is still a erry usefull technique.
For example in the screenshot image in the doc page, I used a chromakey to get a first matte (which is not really good) then I dilate it and use his invert as a garbage matte.
Hope that help.