It depends. In this case, world.Broadcast is buffered by 10 items, so a backlog of up to 10 items will be sent in a non-blocking manner. If the backlog gets full, sends after that are blocking, since they have to wait for a spot to be free before they can send.

In typical usage though, it should be non-blocking (otherwise we should move stuff around so that thread is not so overloaded).