I believe the person calls you via the GV number, then you answer the phone, and mid-conversation, when you want to switch, you press a key; when you press a key, all your GV registered numbers/devices ring; you pick up the one you want and the conversation continues on it.
The major limitation, AFAIK, is that the call has to start with GV -- that is, the person has to call your GV number, or perhaps you can call out from the menus in GV. If I understand correctly, they cannot call your real cell phone number, for instance -- GV wouldn't have a connection to the call and can't forward it.