It seems to be based on Photoshop’s “luminosity” calculation (Adobe’s invented word for what video people call “luma”) – in other words a weighted sum of the numerical values of the three channels. See http://en.wikipedia.org/wiki/Luma_(video)
This is sort of silly, since the actual effect of the composite channel has nothing to do with “luminosity”, but instead acts on each channel independently, and therefore unless a pixel’s color is a neutral gray, the curve will affect each of its component values independently. The clicked-on pixel might not be at the chosen point on the curve in any of the three components.
Basically, think of it as just a relatively arbitrary point that doesn’t bear much relation to the effect the curve will have on the clicked pixel, unless that pixel happens to be gray.
Europe, Middle East and Africa