The User option is taken into account only when Abyss Web Server is run as root. In such a case, the web server changes its executing user to what is specified in the User option immediately after Abyss Web Server is launched. Subsequent restarts do not affect the executing user even if the value of the User option changes meanwhile. To modify the executing user after you modified the User option, you must halt the server and launch it again.
The server cannot be run as root for security considerations. But you can override this limitation by setting User to root and adding the -r switch on the command line when launching Abyss Web Server.
If you run Abyss Web Server as a normal user (not root), it ignores the User option and continues its execution without changing the user.