From Apple’s App Preview page on their developer site:
With iOS 8 and OS X Yosemite, you can capture real-time footage of your app directly from your iOS device. Simply connect your device to your Mac using the Lightning connector and it will be automatically available as a video camera. You can capture anything you’re doing on-screen directly to your Mac using QuickTime Player.
This is a handy little tip (albeit dated at this point) that’s worth keeping in mind when I finally upgrade to Yosemite. I still have no concrete plans to upgrade, but I’d be willing to bet I’ll finally make the switch once Apple releases Photos for OS X.