This option tells FWEAVE to display detailed reductions of the scraps as it does the pretty-printing. (For a discussion of scraps, see -1.) Sometimes FWEAVE fails spectacularly at pretty-printing a section, either because of a syntax error on the part of the user or because of a bug in FWEAVE’s logic. This option helps one (usually the system developer!) to figure out why.
This feature can be turned on more locally by means of the ‘@2’ command. See AT2.