The flash storage on Exadata is part of the storage cells - it is not related to SGA. It is used primarily by the Exadata smart flash cache feature to cache hot blocks in flash cache instead of needing to go to disk to retrieve them. This is all done dynamically and transparently to the database. Although you are allowed to manually control use of the flash storage by pinning tables/partitions to the flash cache or creating flash disks. With the new features in X4 allowing the smart flash cache to even use the cache for full scan operations there is much less need to pin objects to the flash cache. The white paper Srini directed to you is an excellent source of more detailed information.
The only other thing I would mention - and the only real use I've seen for flash disks - is using flash for temp operations. Even after effective indexing and tuning if your environment is bottlenecked by large temp operations, from things like large hash joins, you may consider evaluating creating flash disks to use for temp tablespace and speed up temp operations.