The server is booted via the blockStar Client. The blockStar Client sifts through a list of the hosts clients. Each time a client is called, blockStar asks the server if the user is asking to open the server. If the server says yes, the server will be launched. If not, go to the next client.
Clients will have to have a mod placed in their server, of course. You simply type /blockstar AuthCodeHere in, and you will never have to worry about it again. Your server will do the rest.