When in SIMPLE recovery model a database log records in the active VLFs. These records are retained, as they may be required for a rollback operation. However, the inactive VLFs are truncated when a checkpoint occurs, meaning that the log records in these VLFs can be immediately overwritten with new log records. This is why a database operating in SIMPLE recovery is referred to as being in auto-truncate mode. In this mode, no "history" is maintained in the log and so it cannot be captured in a log backup and used as part of the restore process.