535fb36d_Using_ALTER_INDEX__Description
[Setting] SORT_IN_TEMPDB [Values] ON | OFF [Description] 
If on, the index will be sorted in tempdb rather than the same DB as the MDF. This only applies if the index is so large it will not fit in the current available memory. It may speed up the rebuild if the tempdb disk is fast and on a non busy disk
Using ALTER INDEX
Description