The point of it
The idea is to make a key word listing viewable in the WordList tool. Key words are a subset of the whole set of words in a word list after all. The display will show the frequencies of the key words without other columns in a key words display.
How to do it
With a saved key word list on screen, choose the menu option Save as | Word list (LST).
The word list will contain all key words which are not negative.