Yeah, this technique just ties into PHP's own session handling by telling php to use your own session handling functions unstead of PHP's.
By setting "session.gc_maxlifetime" you tell PHP how long a session should live for and by setting 'session.gc_probability' (to 100) you tell essentially tell PHP to run the session garbage collection function every time a session is accessed.
What's really nice about writing your own session handler is that in theory you can tell your session handler to set properties that would be expected by other applications like PHPBB. I don't know if this would work yet. I haven't tried it but I don't see why it wouldn't. Then you would just have to tell the other app to call its session the same as your plog session. It should allow you to have a single login (through Plog) for any other apps you install in your site.