Primary Color Correction: Tonal Correction Tools - The Art and Technique of Digital Color Correction - page 48

Fig. 2.17
That's because all of the pixels to the left of the black triangle on the input side have “piled up” at pure black
on the output side ( Figure 2.17 ). The more technical description of what's happening is that the levels are being
remapped; all of the pixels in the source footage between 0 and 40 have now been remapped to 0. And all of the
pixels between 40 and 255 have been remapped between 0 and 255. So all of the levels of tonality that are in the
slope to the left of the input black triangle have been compressed, or clipped down to black on the output side. This
means you have lost all the detail in the darkest pixels, because where there used to be subtle differences between
these very dark pixels, now all of them are at 0. Usually this “crushing” of detail is a bad thing, but it can also be
useful to create a punchy, crushed black look. As long as you don't mind losing the detail in the deepest black areas,
you're fine. But if you want to preserve that detail, then stop before the big spike gets too big.
Fig. 2.18