Photoshop Levels
Input Levels Clipping Preview

Black and White Point Clipping Preview

One of the features of Levels is its ability to show what pixels are being remapped to tone 0 and tone 255 when using Input Levels.  In Figure 1 is an image we will use in the following examples.

With the dialog box open, press the Alt (Option) key while dragging either the black Input Levels slider or the white Input Levels slider.  Holding down the Alt (Option) key turns the image in the document window into a Threshold-like display called the clipping preview.  In the clipping preview, all pixels are temporarily displayed as either solid black or solid white.  As we move the black or white sliders while pressing the Alt (Option) key, those pixels that will be remapped to tone 0 or 255 will appear in the clipping preview.
 

Referencing Figures 2A and 2B; holding the Alt (Option) key while moving the black Input Levels slider causes the image to go solid white.  As the black slider is moved, those pixels that would get remapped to tone 0 turn black in the document window.  The more the black slider is moved inward, the more pixels will be remapped to tone 0 if the black slider were left in that position.  If there is any detail in these areas, it will be lost.  Once the Alt (Option) key is released, the clipping preview will disappear.

Referencing Figures 3A and 3B; holding the Alt (Option) key while moving the white Input Levels slider turns the image solid black.  As the white slider is moved inward, those pixels that would get remapped to tone 255 turn white in the document window.  The more the white slider is moved inward, the more pixels will be remapped to tone 255 if the white slider were left in that position.  If there is any detail in these areas, it will be lost.  Once the Alt (Option) key is released, the clipping preview will disappear.