[DutchDaemon]

See the examples in that same manual ..

EXAMPLES
     su -m man -c catman
            Runs the command catman as user man.  You will be asked for man's
            password unless your real UID is 0.  Note that the -m option is
            required since user ``man'' does not have a valid shell by
            default.
     su -m man -c 'catman /usr/share/man /usr/local/man'
            Same as above, but the target command consists of more than a sin-
            gle word and hence is quoted for use with the -c option being
            passed to the shell.  (Most shells expect the argument to -c to be
            a single word).

And:

su -m www ps    
/bin/ps: /bin/ps: cannot execute binary file

su -m www -c ps  
  PID  TT  STAT      TIME COMMAND
 1023  p0  R+     0:00.00 ps