Will svn: external subdirectory be in sync with its HEAD version every time svn is updated for the parent working copy?

In this page in the Subversion manual , it says that "When you make a change to the svn: externals property, Subversion will sync the crossed out items to match the changed external parameter definition the next time you run svn updates. The same will happen when others update their working copies and will receive your changes in the external definition. "

Unless you provide a version number for the external one, I assume the HEAD revision is being used. So, will a subdirectory controlled by updating an external property be revised HEAD every time a "svn update" is performed against the parent working copy?