The small triangular faders
directly below the histogram
display are used to set the tone value spread.
The black one represents the darkest tones (black) and the white one the lightest (white). For example, if you pull the black fader 20 steps to the right, the resulting grey value is now defined as a new black, and values from that point up to the white fader are spread out to the full 256 grey levels.
These faders are meant primarily to increase the contrast, as for instance certain dark grey values can be set to black, or light ones to white, and the entire tone value range is distributed evenly over the section in between the two.
The small grey fader provides gamma correction. It provides a simple way to adjust the overall brightness. A move to the left darkens the image, while a move to the right lightens it.