Backup Log WITH NO_LOG, Change Rec Mod. To SIMPLE?

Hi,

I use FULL recovery model, SQL 2005. Is it possible this type of
backup ""change"" my recovery model to SIMPLE. I noticed when I
executed this:

BACKUP DATABSE db_name
TO DISK = 'path'
BACKUP LOG db_name WITH NO_LOG
DBCC SHRINKFILE ('db_name_log', truncateonly)

Now transact log grow very, very, very slow (this is symptom simple
model). But when executed this (different order):

BACKUP LOG db_name WITH NO_LOG
DBCC SHRINKFILE ('db_name_log', truncateonly)
BACKUP DATABSE db_name
TO DISK = 'path'

transact log grow normally

Would somebody explain me this, and tell me first statement change
(theoretically) my model to SIMPLE?

--
Regards,
anxcomp