Actually, EMC does a better job explaining how to use BCVs with SQL Server than I can. We've never gotten them to work to my satisfaction.
The basic idea is that a BCV will allow you to mirror data in real time. In SQL 7.0 hooks were added, which were refined in SQL-2000 to allow an application to "freeze" a disk volume. This freeze would effectively flush the disk buffers to disk as of a specific point in time, almost as though the database had been stopped (to detach it). The theory is that you could mirror a BCV volume to a production volume, freeze the SQL files, break the mirror, then "thaw" the SQL files again, all without the end user knowing or caring that anything had ever happened.
Anywho, see the EMC document for a better description. While I love EMC in general, we (my firm working with EMC) were never able to make BCVs work in production.