the mainserver works (but it's only my opinion) only to make a first contact. just to see, is the peer can connect withoun any trouble & stuff like that .. what i want to say it's simple, just like in hamachi, it's check, can u connect. do u have fully connection. is there is some problems (like unable to make secure connection/n vpn connection/otthers) and after that, when the server is sure all works fine, it's sending a message to other ppl's in you network with all information he can get.
but yes, if the main server is off, u cannot connect to other's .. it's not wippien
but it's only what i think and this does not need to be wright ;]