Well I think he refers to the trick to use a Phonebook card (which is used to copy contacts)... before aug5 it use to let you bypass the activation screen and you could run and restore to older firmware, i don't know if using the Phonebook card would still work, they usually go for like $5.... (The best would be to get either a prepaid card of the original company).
There is such a thing as lockdown the location is here: /var/root/Library/Lockdown/activation_records/wildcard_record.plist
And people replace the wildcard plist when they put another sim and the phone tells them "different sim detected" so they go through SSH and change it to the other plist that was used in the older jb... these people can receive calls and txt, so they just remove the sim card, they go to the homescreen, cydia, ssh and then replace to the older plist form previous jb