Non-blocking operation to back up MyISAM tables using Percona XtraBackup?

On the page it mentions the following:

I am using MySQL 5.6.30 and Percona XtraBackup 2.3.4 to backup a MyISAM table of approximately 32 million records but I am only experiencing a full table lock while the backup operation is happening. So it’s less of a brief pause at the end of the backup and more of a long pause throughout the entire process.

Is there a flag I need to pass into XtraBackup to achieve this functionality? The command I’ve been using so far is as follows:

Thank you in advance for your help!