Hi there
That's the response I got from an DS official:
Hello Martin,
The Bounce back feature:
Is a scheduled application (Under host/Schedule) and it actually logs in and checks the an email and searches each email for some matching keywords such as undeliverable, bounced, auto reply, etc.. Then it checks the email address on that user and matches it against the opt in email list.
If you have any further questions please let me know.
Thanks,
Ryan
Greetings,
Martin