OK now I have tried:
<action application="set" data="enable_file_write_buffering=false"/>
<action application="eval" data="${uuid_record(${uuid} start /recordings/firstTest.fsv)}"/>
But I receive errors:
Error finding the folder path section in /recordings/firstTest.fsv
Error invalid file format /recordings/firstTest.fsv
Error opening file /recordings/firstTest.fsv