Well, I'd have to test it, but here's the theory. I don't want to forward the initial incoming calls to the boss's extension, just calls that have been already answered (by the auto attendant?) and transferred to the extensions. So without any lines assigned to the extensions, the calls would end up ringing on the intercom buttons, and the extensions being forwarded, it should follow the forward.
However, with no lines assigned to them, there is nothing to ring on the initial incoming call while waiting for the auto attendant or operator to answer the call.
I originally discovered the forwarding action when I needed to plug in my test phone and do some programming while back at the control unit. This customer had the auto attendant answering all incoming calls. I needed to use ext 11, leaving ext. 10 alone. The phones were set to no ring. I pulled ext 11 out, plugged in, did a quick copy (#399) of Ext. 11 to another port, and moved the connecting cord for 11 to that port. Now, this port was also set for no ring (copied from 11), and I forwarded 11 to this port (Feature 11-11-XX), and all of a sudden, incoming calls start ringing there before the auto attendant could answer!! I still don't know "why" they designed it that way, but it's surely a gotcha.