Parititions, global indexes - The magic "update indexes" clause

Global indexes need not go invalid when you change a table's partitioning if you use the magic 'update indexes' clause. Downside, it takes longer to perform the operation. Upside, users can continue to use the table without getting the error 'index is unusable'

All explained here :-

http://youngcow.net/doc/oracle10g/server.102/b14231/partiti.htm#i1006455
in the paragraph "Updating Indexes Automatically"