This is very useful for copying say your Live instance to Staging or another tier. Doing a disconnected duplication means there is absolutely no chance of upsetting your Live instance. I also think it’s more intuitive. (It’s only available in 11g.)

To begin, you need a backup of your source instance, including control file snapshot and any archive logs you want to include, and transfer those to the destination server. (The copied backup files don’t need to be in a particular structure and don’t need to be in the same directory.) I rsync the files I need into /opt/oracle/tmp/{backupset,archivelog,autobackup}/2011_06_21/.... The gotcha I need to remember is that archivelogs need to be gunzipped before the first attempt at import.

If you’ve taken care of all the steps for creating the destination instance, you’re ready to kick off the import.

sqlplus / as sysdba
startup nomount
rman auxiliary /
duplicate database to DEST
until time "TO_DATE('2011-06-21 05:00:00', 'YYYY-MM-DD HH24:MI:SS')"
backup location '/path/to/tree/of/backup/files';

(where DEST is the name of the destination instance.)

sqlplus / as sysdba
alter database open

This often fails for one reason or another. Simply re-run the rman step and it should work second time. This will import everything under /path/to/tree/of/backup/files up to the specified cut-off time.
You may then want to disable archive logs if you don’t want them on the new instance.
Most likely you’ll also need to rebuild your TEMP tablespace(s).