When the jar command is run without arguments, it prints some help, at the end including this example:

Example 2: use an existing manifest file 'mymanifest' and archive all the
           files in the foo/ directory into 'classes.jar':
       jar cvfm classes.jar mymanifest -C foo/ .

Including the '/' in "foo/" is worse than confusing: On windows, it causes the resulting jar to be created as if the -C had not been given.