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.