In iOS 5, Apple introduced something they dare to call multi-tasking. Unfortunately, they stripped the end user of the right to choose which application will and will not work in the background. And all this backgrounding is gone as soon as you lock your device. Don’t bother, just another stupid Apple limitation.
There’s at least one app you would like to continue running in the background. That’s a torrent client. However, you actually can force it to continue file transfer even when the screen is locked. Or at least try.
The first thing you need is Backgrounder and LastApp utilities from Cydia. Install them, then go to Settings and tap LastApp. Enable Backgrounding and exit to the desktop, where you launch Backgrounder by taping the icon.
Select Overrides, tap Add, include a torrent client, click on it, switch Forced, Enable at Launch and Stay Enabled to on.
You’re done. Now when you start the torrent client, it will continue to download and upload data even if you leave it, which is indicated by a sun rays icon in the top right corner of the status bar. If the icon is a N (Native), that means the application is running in native mode and you have to setup Backgrounder as described above to force it run in the background.
To completely stop a backgrounded application, double press the Home button or put 4 fingers on the screen, then hold the torrent client icon until it starts to shake. Now press minus in the corner of the icon. This will completely unload an app from the device memory.