When would <event>.queued be fired?

From this article,

https://blog.dreamfactory.com/queueing-with-dreamfactory-scripting/

there are 3 events that can be fired when running GET

api/v2/db/_table/<table_name>.get

I understand the Pre-Process and Post-Process. But I can’t figure out when and how .queued is fired.

Can someone help and explain this?

Thanks a lot!