Adobe 65016169 - Photoshop CS4 Extended Tutorial page 26

3. Make sure the Preview box is checked off. Choose the channel you wish to change
and drag the triangles. Dragging the black triangle to the right will make the
shadows in your photo darker; dragging the white triangle to the left will make the
higlights in your photo lighter; dragging the gray triangle to the left will make the
midtones in your photo lighter, dragging it to the right will make the midtones darker.
You will be able to see the changes in your image.
Adobe Photoshop CS4 p. 26