Thechroma key technique is just based on the chroma diference betwem an image and a reference, this permit todo 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 grenscren or bluescreen.
A chromakey is an histórical method form the análogous 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 ned todo despill, Color Correct and composite (with Edge blending blur better) this Will ned a Lots of nodes (arround 50) with a lot of redondant/unsused parameters.
My node does all that internally and permit you todo choose diferent method for some operación (no more confusión with mixing the nodles conexións).
If you can put your hands on digital compositing for film and video by Steve Wright, y deply suggest it todo you, it explains all the keying techniques and much more.
Also, if you dont 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 bien todo do grenscren or bluescreen, but it is still a erry usefull technique.
For example in the screenshot image in the doc page, y used a chromakey todo get a first matte (which is not really god) then i dilate it and use his invert as a Garbage matte.
Hope that help.