“to place a light behind … the canvas … hold the alt-key while dragging in this window”
That’s what I am told in the light-palette when I hold down the ctrl-key.
Problem: It does not work and has never worked for me. (currently: Mac OS 10.8.2: ZBrush 4R5; MacPro, ATI graphics; Macbook Pro Retina, NVidia Graphics …).
Regardless which button I hold down while dragging or adding a light I simply can not manage to place a light behind a mesh in 3D-mode. No keyboard-button toggles anything …
What’s the issue or the deal here? Do I miss some major important and or unintuitive thing here?
If you need images or videos for demos of this bug, just let me know.
Thanks, Olaf

Although it is completely the opposite of what the interface suggests doing. Where can/should I bug-report?
… and they fix these issues soon. ZBrush is so great, but if neither books (old) nor the help-system (should be up-to-date) help(s), it is very difficult to develop a deep love for it.