When does CLUSTER occur?

Hi,

I'm looking for some general info on the behavior of CLUSTER. I add large amounts of data to an existing table something like this:

BEGIN;
DROP INDEX IF EXISTS xgen1011_si_sn;
COMMIT;

BEGIN;
INSERT INTO gen1011
[snip]
COMMIT;

BEGIN;
CREATE INDEX xgen1011_si_sn
ON gen1011
USING btree
(sample_id, snp_number)
WITH (FILLFACTOR=100)
TABLESPACE index_tablespace;
ALTER TABLE gen1011 CLUSTER ON xgen1011_si_sn;
COMMIT;

My question is whether or not the "ALTER TABLE gen1011 CLUSTER ON xgen1011_si_sn" actually clusters the table at that point or if it just tells it to use that index for clustering? If the latter I assume I need to add a "CLUSTER gen1011 ON xgen1011_si_sn" line at the end along with an ANALYSE line?

Thanks,
Bob