Flash mode is a mobile phone software program, used by developers, to make changes in a smartphone. In Apple iPhone, this flash mode is known as DFU mode.
Flash mode is an environment of the mobile phone where it does not have any operating system. When in this mode, the cell phone works like a plain Java phone or just a terminal. In this mode, you can carry out different supported programming tasks. One can downgrade or upgrade the firmware and carry out other tasks too. It's called the Device Firmware Upgrade (DFU) mode in Apple terminology.
If you are trying to restore iPhone firmware or downgrade it, you need to put it in DFU mode. Also, if the iPhone freezes or one needs to restore and save valuable data on it, DFU mode is the last resort.
Manually Putting the iPhone in DFU Mode
Connect the iPhone to your computer. Turn it off and open iTunes on your computer.
Hold the power button for about 3 seconds.
Then you need to hold the Power and Home button simultaneously for about 10 seconds, and allow your phone to boot.
Release the Power button, but continue holding the Home button.
Hold the Home button till you observe a pop-up appear in the iTunes window on your computer. This pop-up will say 'iPhone detected in Recovery Mode'. You need to press OK to continue.
Once your phone is in DFU mode, you will observe a black screen on the display. You may even notice a small icon that looks like a USB cable pointing towards a hard drive. The occurrence of this feature depends on whether or not your phone is jailbroken.
You can try the above tutorial at your own risk, only if you are confident of carrying out the process successfully.