Oracle Database Discussions

Hi everybody,
i need to separate data from index so i decided to move (rebuild) index to a new tbs.
i use the command:
alter index index_name rebuild tablespace idx_tbs online;

So far so good..
But i'm curious.. what is oracle doing to accomplish that?

I run a query on dba_indexes and noted a strange thing..
while rebuilding the first index the others are marked as unusable.. Whoa!
I suppose they should be valid until the new one is ready!

Am i missing something?
I'm using a 10g 10.1.0.5 to do my tests..

Any ideas?

Thanks
Acr