This is somewhat rare, but if an i OS update is interrupted it can happen, which usually requires a device be placed into DFU mode and restored manually with firmware.
Typically a normal i Tunes based restore is sufficient, which is why that is fairly rare.
If you’ve recently downloaded an i OS IPSW file, you might be curious how to use it to update your i Phone, i Pod touch, or i Pad.
In 2005, when Steve Jobs began planning the i Phone, he had a choice to either "shrink the Mac, which would be an epic feat of engineering, or enlarge the i Pod".
Jobs favored the former approach but pitted the Macintosh and i Pod teams, led by Scott Forstall and Tony Fadell, respectively, against each other in an internal competition, with Forstall winning by creating the i Phone OS.
The i OS user interface is based upon direct manipulation, using multi-touch gestures.
Interface control elements consist of sliders, switches, and buttons.
Finally, there are some cases where IPSW is necessary to create custom configured firmware for future restores.
This typically ties in with the “Restore” method of updating, which allows the use of custom IPSW files, and is generally something created with a third party application like Pwnage Tool or sn0wbreeze.Interaction with the OS includes gestures such as swipe, tap, pinch, and reverse pinch, all of which have specific definitions within the context of the i OS operating system and its multi-touch interface.Internal accelerometers are used by some applications to respond to shaking the device (one common result is the undo command) or rotating it in three dimensions (one common result is switching between portrait and landscape mode).Initially, third-party native applications were not supported.Jobs' reasoning was that developers could build web applications through the Safari web browser that "would behave like native apps on the i Phone".As of January 2017, Apple's App Store contains more than 2.2 million i OS applications, 1 million of which are native for i Pads.