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.

GeoHot the most talented iOS Hacker out there has once again surprised the world by releasing his brand new iOS 4.0-4.1 Jailbreaking Tool - "Limera1n", exploiting a previously unreleased bootrom-level hole GeoHot's Limera1n is capable of jailbreaking iPhone3GS, iPhone4, iPod touch 3G, iPod touch 4G, the iPad, and technically even AppleTV.