There’s a lot more info on the project’s homepage.

A common way to transfer a file is to first upload data to a server, and then, once uploaded, the link is shared with someone who goes to download it. This method is easy, but slow – the transfer rate of the file is half the harmonic mean of the upload and download speeds which makes it slower than either just uploading or downloading. There are better ways than sequentially uploading and downloading. instead, you can use a relay server to create a full-duplex real-time communication layer between the two computers so that “uploading” and “downloading” occur simultaneously between the two computers. this effectively increases the transfer rate because its not sequential and only limited by the slower of the two transfer modes (uploading or downloading).