As a relative newbie to the Spectator crossword I’m sometimes baffled by the complications involving unclued solutions, and 2721 gives a particularly obscure explanation. There are obviously more than 21 “blanks”, I don’t see why some of the letters offered up are upper case unless they are the names of places etc., and what does “jig-saw fashion” mean? 
I understand what “In short” implies but am none the wiser….
Help!