This minor mode allows functions to operate on the current line if they would normally operate on a region and region is currently undefined. The primary use for this is to kill (cut) the current line if no region is defined, and kill-region is invoked. It basically saves you the effort of going to the begining of the line, selecting the text up to the end of the line, and killing. Similarly, when yanking, it's smart enough to know that the string to be yanked was killed as a whole line, and it should be yanked as one, too. So you don't need to position yourself at the start of the line before yanking. If region *is* defined, though, all functions act as normal.