I read the following:

Changes to "Persistent" instances are detected at FLUSH TIME and result in a SQL [UPDATE] ---> [saveOrUpdate()].

What is meant by the "Flush time"?

Thanks.

flush time is the point at which the engine decides that it need to synchronize your changes to the db and actually execute the sql.

i.e. you could call session.save(object1), session.save(object2), .....session.save(objectn) and if you were tracing the sql you wouldn't actually see any inserts executed.

However, if you were to turn around and select from that table with FlushMode.AUTO on then the hibernate engine would detect that you have changed entities in the table set you are querying and trigger the insert sql prior to executing the fetch. The same sort of behavior can be forced by manually calling the flush.