The default iOS multitasking works by suspending the process in the memory. However with the Background Manager, you can choose to run any app on the background as if it was running on foreground.
Notably, Background Manager is not an update to the popular Backgrounder tweak; rather, it's a tweak that was completely written from scratch to bring Backgrounder-like functions to iOS 6.
The latest update to Background Manager fixes issues with app crashes, apps not loading, and badge numbers. Preferences are configured from the Settings app.
You can purchase the tweak from the Cydia Store for $0.99.