When does pg_archivecleanup work?

As I have very low wal_keep_segments compare to my wal generation, I am
collecting archive wal files at slave.
Now in order to clean up archive wal collection directory at slave, I used
"archive_cleanup_command".
I watched that after archive wal files were pilling up at slave and after
certain point it was cleared up.
But still I didn't get the idea when it will be executed and clean up my
directory.

Example: recovery.conf that I used:
standby_mode = 'on' # enables stand-by (readonly) mode
primary_conninfo = 'host= 10.0.0.1 port= 5432 user= replicator
password=replicator'
trigger_file = '/pgdata/pgsql.trigger.5432'
restore_command = 'cp /archivewal/%f %p'
archive_cleanup_command = '$HOME/bin/pg_archivecleanup /archivewal %r'

Any idea please.

Thanks.