When using VALIDATE option, Oracle will validate backup and not create backups and will not restore when this option is used.

# Validate if datafile exists and have no physical or logical corruptions that would prevent RMAN from running. If “archivelog” is specified it will also check if archive logs are valid too
RMAN> backup validate database archivelog all;

# Validate if datafile exists and have no physical or logical corruptions that would prevent RMAN from running.
RMAN> backup validate database;

# Validate if a specific tablespace can be restored, Oracle will check the backup piece to validate if the tablespace can be restored
RMAN> RESTORE VALIDATE TABLESPACE system;

Starting restore at 31-JAN-11
using channel ORA_DISK_1

channel ORA_DISK_1: starting validation of datafile backupset
channel ORA_DISK_1: reading from backup piece /u01/oradata/TEST/rman/60m3efef
channel ORA_DISK_1: restored backup piece 1
piece handle=/u01/oradata/TEST/rman/60m3efef tag=TAG20110130T203006
channel ORA_DISK_1: validation complete, elapsed time: 00:00:47
Finished restore at 31-JAN-11

# Validate backup for database full restore, RMAN will not perform restore it will validate backup piece
RMAN> RESTORE VALIDATE DATABASE;

Starting restore at 31-JAN-11
using channel ORA_DISK_1

channel ORA_DISK_1: starting validation of datafile backupset
channel ORA_DISK_1: reading from backup piece /u01/oradata/TEST/rman/60m3efef
channel ORA_DISK_1: restored backup piece 1
piece handle=/u01/oradata/TEST/rman/60m3efef tag=TAG20110130T203006
channel ORA_DISK_1: validation complete, elapsed time: 00:00:36
Finished restore at 31-JAN-11