BOL refers In the full and bulk-logged recovery models, a sequence of transaction log backups is being maintained. The part of the logical log before the MinLSN cannot be truncated until those log records have been copied to a log backup.
If backing up the log does not appear to truncate most of the log, an old open transaction may exist in the log. Log space can be monitored with DBCC SQLPERF (LOGSPACE).
After backing up the log using either NO_LOG or TRUNCATE_ONLY, the changes recorded in the log are not recoverable. For recovery purposes, immediately execute BACKUP DATABASE for safe recovery.