FAST_START_PARALLEL_ROLLBACK specifies the degree of parallelism used when recovering terminated transactions. Terminated transactions are transactions that are active before a system failure. If a system fails when there are uncommitted parallel DML or DDL transactions, then you can speed up transaction recovery during startup by using this parameter.
Parallel rollback is disabled
Limits the maximum degree of parallelism to 2 *
Limits the maximum degree of parallelism to 4 *
If you change the value of this parameter, then transaction recovery will be stopped and restarted with the new implied degree of parallelisme.