When you are running an application on your local Mac from a remote SSH tunnel (via ssh -X), and that application needs to spawn windows programmatically, your local XQuartz server will not be able to draw the windows unless you set “Allow connections from network clients.”

We ran into this when running MatLab R2013 (or so) via an ssh tunnel from a remote RHEL6 compute server: the MATLAB script spawns windows that are java figures, and the local XQuartz server by default wouldn’t draw them — although oddly, they did spawn. With this switch set, it works, as shown: