Basically, DFU mode is almost similar to the Recovery mode, But DFU mode in low-level. Changes or modifications to the OS is only possible in DFU mode because the OS has not been loaded yet.
When you are in DFU mode, the operating system is not running, and there is no interaction at all, you'll see a screen off / dim. But even though the display is dead, but Devices still active and can still accept input via USB cable connection with iTunes. iPhone or iPad in DFU mode position, only iTunes can communicate with the devices.
DFU mode is a condition that you can insert into your iPhone and it can interact with iTunes but does not load an operating system or boot loader on iPhone (this difference between DFU mode with recovery mode). DFU stands for Device Firmware Update. For you who are still difficulties in entering DFU mode, do the following:
1. Open iTunes on your Computer Or Laptop, and then make sure your device is connected to the Original USB Connector and still turned on.
2. You need to turn off the device by pressing the power button to shutdown your device.
3. Now, press the Power button for 3 seconds, until the Apple logo appears on the screen of the device. Without releasing the Power button, press the Home button for 7-10 seconds, or until the screen flashes and the Apple logo is missing.
4. Keep pressing the Home button and release the Power button until your device is detected in the iTunes.
Maybe you are a little confused, why in the face of iTunes, your device is detected in a state of Recovery Mode. To make sure that you've actually been in DFU mode, only look at the Display on your device, if there are images about USB Connector and iTunes, that means you failed. But, if the screen on your device does not show a picture or a blank, and the device looks like turned off, it means you're already a success.
In general DFU mode can be used for bypass iOS running, to restore with iTunes without any problems. Also, there are also some other things, which always be a reason why users set their device into DFU mode before doing a restore.