I think it would make more sense for the last sentence to be broken up and the next paragraph to be brought into this one.  Something like:

Each channel receives a *copy* of all messages on a topic via push. The topic needs to *push* the message to each channel instead of relying on each channel to receive the message from topic's go-chan.  The push is what *distributes* the messages.  In the common case of a topic with a single channel, the message has a fast-path that avoids copying the message struct.  Because a slice is a reference type, copies do not duplicate the message body, only the metadata.