Expanding allows you to show all of the area that was recorded instead of an area the size of the inputimage based on the alignment-feature. The image to the left is the normal output and the image below shows the expand effect. In this case the image drifted SE-NW during the recording. For proper expanding It is important that the alignment feature is visible on all images in a sequence.  Also on the edges of the image more noise will become apparent as they are “covered” by fewer image-frames.