Partial coverage compensation
This option during stacking is usefull when the shifts in the sequence are clearly visible. By default the option is ON. In the above image it was put off and on the right of the image a darker section can be seen. The pixels in that section were not visible in all the frames that were used in the stack. Using Partial coverage compensation the pixelvalue in the stacked image will be calculated based on the amount of frames that cover that area. The image below clearly shows the effect of this. Another more dramatic example can be seen at this page