Comment 42 for bug 1054299

That is handled in xflock4 I attached in https://bugzilla.xfce.org/show_bug.cgi?id=10217 :

If such a fallback locker is used, they will be called in background in xflock4. If you have defined such a locker in xfce4-session -> /general/LockCommand configuration variable, you should not set xfce4-session -> /general/LockWaitComplete to true. Otherwise, when you suspend (with expected locking) it will just lock, but suspends only after you unlock. If xflock4 expects a locker not to return before unlock, it will be run in background and its success is determined by checking, if it is still running after short delay.