PkgBackup saves everything by creating a fake contact in your contacts directory and basically just encodes the list of apps and packages into a hash that is then stored as the contacts name. Once the "contact" is created, they advise you to sync your phone to iTunes in order to backup the contacts list. Then, when you restore, update, etc. you simply reload your contacts list, jailbreak, and then install PkgBackup from Cydia like before, and then go into the app and it will identify the "contact" and allow you to install all of the apps synced to that contact "name" at once without having to go back and forth through Cydia.
Honestly, unless you really need it immediately, I would hold off on buying it as it is $4.99. The forthcoming Cydia update should have a built-in feature for reinstalling previously purchased apps. There has not been an ETA given for that update, however.