The SORT_IN_TEMPDB option directs the index creation processing to the tempdb database, instead of the database in use.
This option may increase rebuild performance for large tables on a high activity environment, if the tempdb system database is located on a separate disk. Although the index creation time may decrease, the disk space requirements increase while using this option.
CREATE NONCLUSTERED INDEX AddIndexNmHere ON
WITH (SORT_IN_TEMPDB = ON)