The DFU (Device Firmware Update) mode is needed to update/upgrade/restore an Apple iDevice, however if you want to flash a custom .IPSW file you must enter a Pwned DFU mode i.e. a special DFU mode which had exploited limera1n jailbroken core to allow operations not permitted by the normal Apple DFU core.
Here is a simple to follow, visual tutorial on how to enter the Pwned DFU mode on an Apple device using ever handy redsn0w jailbreaking tool.
- Download the latest version of redsn0w for your operating system from the official download location - https://sites.google.com/a/iphone-dev.com/files/
- Launch redsn0w and click "Extras".
- Now click "Pwned DFU" button.
- Follow the onscreen instructions to get your device into normal DFU mode as instructed (if not already in DFU mode).
- Once in DFU mode redsn0w will start the limera1n exploit upload to get into the PWNED DFU MODE.
- Once this gets completed your iDevice screen will be black and you should have sucesfully entered the Pwned DFU mode.