When you start a passive node backup, the replication service notifies the information store service to prevent the same storage group on both the active and the passive nodes from being simultaneously backed up. It also notifies the Store it when the backup task is finished. Once the transaction logs are copied onto the passive node, the active node transaction logs are deleted.
This process is more complicated than traditional backups. In Exchange 2000 and Exchange 2003, right after the backup process was completed, the passive node database header would reflect the backup. With CCR, the active node database header would reflect the backup, but backups to the passive node aren’t reflected on the passive node database header until the transaction logs are played into the database. For further information on this topic, see technet.microsoft.com/en-us/library/aa998870.aspx.