The official user manual explains only the normal offset operations. There is no information regarding the back offset plotting. I recommend you contact the official developers for further details. You can get in touch with them using
091-11-2430 5166 phone number or
firstname.lastname@example.org email address.
Note: The official web page was marked as potentially unsafe, but you can ignore that warning and add the page on the exceptions list, because it doesn't have any security problem.