from the BeInSync Technical Overview, http://www.beinsync.com/info/technical_overview.php
BeInSync create an on-the-fly private peer-to-peer network that spans your colleagues, friends, computers and online storage. Once established, BeInSync makes sure your photos, videos, music and documents are always available and up-to-date wherever you need them:
* Files and folders are efficiently synchronized so that they are available and up-to-date on all your computers. Synchronization is peer-to-peer and uses compression and byte-level file transfer for maximum speed.
* Files and folders can be easily shared over the Web or kept in sync with a group of people.
* Users may access their remote files from any Web browser.
* Files may be kept always backed up to a secure off-site online storage.