xtrabackup --backup --incremental-lsn=11111 --stream=tar /tmp fails to create a incremental backup

I've attempted to generate a incremental backup by specifying the lsn # as a command line option using teh following format.

xtrabackup --backup --incremental-lsn=(lsn#from xtrabackup_checkpoint file) --stream=tar /tmp > backup.tar

This generates the same backup if I do the following.

xtrabackup --backup --stream=tar /tmp > backup.tar

The file is the same size. I've attempted similar commands with innobackupex and have tried the 1.6.x and 1.9.x versions without success. The xtrabackup help shows the following for incremental-lsn.

--incremental-lsn=name
                      (for --backup): copy only .ibd pages newer than specified
                      LSN 'high:low'. ##ATTENTION##: If a wrong LSN value is
                      specified, it is impossible to diagnose this, causing the
                      backup to be unusable. Be careful!

not sure what LSN 'high:low' means. I tried 0:111111 and 111111:0 and both return errors about the lsn being in the wrong format. If I don't sue --stream=tar and backup up the db's to a directory I can create a full and then an incremental at the same time. Just don't want to send it all to a directory first if I can send it to a file. Which we prefer.