The comment in the distributed transition.ini is : "Thumbnails desaturation in tasknav"

thumb_desaturation = 1 means effect on
thumb_desaturation = 0 means effect off

When activated and you close an app (only in the thumbnail view), the application will be grayed-out until it really is gone.

It has to indicate that an application has received a kill signal in case exiting the program takes some time.