I haven't had a chance to check out the craft ingame but I think I understand how it works. Since parts that are part of the same vessel can't interact with itself (except for landing legs), the "traditional" way of moving and locking a part would require engines/landing struts and docking ports-- which can't undock via action groups. Your ingenious solution to this is to have a second, structural vessel that allows the main spacecraft to interact with itself by giving it something to push on while in turn the "shell" interacts with the engine assembly that's attached to the hinge, causing the entire assembly to move. Absolutely brilliant!
BTW the Z-50 (now the Z-51) is still available here: http://forum.kerbalspaceprogram.com/index.php?/topic/36708-zepto-eagleworks-z-51-cobra-vstol/#comment-505173