The Histogram looks at your footage and maps out all the values along a 0-255 tonal scale (where 0 is full black and 255 is full white).  By adjusting any of the three sliders you can adjust the tonal values that dVeloper will output when rendering.  You will notice the histogram changes when you change your Region of Interest.  The white, gray and black settings correlate directly to the histogram by giving you the actual numbered tonal values (0-255). These numbers can also be changed by highlighting and typing.

The Gamma Tension correlates directly with the blue line that runs from left to right, bottom to top in the histogram. The blue line represents the tonal values that are being output by dVeloper. If you drag any of the level sliders in the histogram and watch the blue line as you do so, you will notice that certain values are being "clipped" and removed from the output of the video. The Gamma Tension adjuster lets you bring those values and details back into the image.