That's a nifty but subtle and tricky solution. Normally, when a shell command ends with redirection to a file like that, and the same file name is also used as input at a previous step in the same command line, the file gets truncated first (the redirect/truncation is processed before the preceding steps are launched), so the the earlier input step ends up reading an empty file.

My mind boggles at how much of the bash man page must be studied to understand why your command line works the way it does, executing the double-paren'd chunk before truncating the output file. And I feel sorry for anyone who looks at that line, concludes that it's a simple and normal thing to use the same file as input and redirection output in a single command, and forgets about the parens.