http://www.freebsd.org/cgi/man.cgi?manpath=freebsd-release-ports&query=rm
[PANEL]
-d Attempt to remove directories as well as other types of files.
-P Overwrite regular files before deleting them. Files are overwritten three times, first with the byte pattern 0xff, then 0x00, and then 0xff again, before they are deleted. Files with multiple links will not be overwritten nor deleted and a warning will be issued. If the
-f option is specified, files with multiple links will also be overwritten and deleted. No warning will be issued.
Specifying this flag for a read only file will cause
rm to generate an error message and exit. The file will not be removed or overwritten.
N.B.: The
-P flag is not considered a security feature (see
BUGS).
-R Attempt to remove the file hierarchy rooted in each
file argument. The
-R option implies the
-d option. If the
-i option is specified, the user is prompted for confirmation before each directory's contents are processed (as well as before the attempt is made to remove the directory). If the user does not respond affirmatively, the file hierarchy rooted in that directory is skipped.
-r Equivalent to
-R.[/PANEL]
So,
-r means recursive delete (it asks
rm to delete the directory and its content), but I have really no idea why Joe suggests
-P...