Threshold adjustment in CMYK - Luminosity in CMYK

I know that it's a pure intellectual question with no real life use  : 

HOW does the threshold adjustment work in CMYK ?

And for those who understand anything about it I think there is the underneath question on how photoshop calculate the luminosity in CMYK as the threshold histogram is in fact the luminosity one ...

( Luminosity in photoshop is ColorSpace independant in RGB but i dont know anything about it in CMYK)

If anybody on this one I am curious