A hidden iOS 9 feature that Apple hasn’t really explained is ReplayKit, and it’s one that many gamers, developers and other iOS users will appreciate. ReplayKit lets you easily record the action from your iPhone or iPad screen, and you don’t even have to wait for iOS 9 to roll out. A cool trick lets you record your iPhone 6 display or any other iOS device right now, free of charge, provided you have a Mac.
As shown in a CNET how-to video, all you need is a Mac running Yosemite and a Lightning cable.
Once you’re connected to your Mac, go to QuickTime Player, then File, then Movie Recording. By default, the computer will use its iSight camera to start recording, but that’s where the awesome trick comes in. Click the little arrow to the right of the record button and select your iPhone, which should now appear in the list.
To add sound to your recording, use the same arrow once again to select your iPhone by name from the Microphone menu.
The video below should offer even more guidance, if you need it.