This app promises to turn your iOS device into a second monitor and uses the standard USB cable to connect to the computer, as opposed to the network which all other similar apps use.
Back when I heard about it, it wasn't yet available so I thought it was a scam. To my surprise, they released the app and it actually works as described.
How does it work ? As far as I know there are no APIs to drive the USB port in iOS, and the computer connected via USB through the standard charging/syncing cable doesn't count as an MFI accessory.
Note that they provide the server software (the one that installs on the Mac and streams the desktop to the device) on their download page for free, reverse-engineering it could shed some light on this but I unfortunately don't have the skills for that.
The information, code and tools to accomplish generic USB port communication is available to those who join the Apple MFi program. You have to do the paperwork and get licensed by Apple.
http://developer.apple.com/programs/mfi/
The libraries necessary are included in the app and then the usual streaming libraries and code do the rest.
来源：https://stackoverflow.com/questions/28124419/how-does-this-ios-app-communicate-over-the-usb-port