See if you can guess what the app External Flash does. Hopefully you guessed “It’s an app that lets you control the LED lamps from up to 16 other iPhones and fire them in sync with your own iPhone’s camera," because then you’d be correct. If you guessed anything else, then you’re totally wrong. Wrong wrong wrong.
The app needs to be a installed on all the phones to work, which makes it a little less useful when you’re in a group. To use it, you put one phone in master mode and set all the others to obey. And that’s about it. Line up your lights, take a shot and all the LED “flashes” fire together. You can share the results to the usual places, and browse the images in app, but really this is useful for its single function – off-camera lighting using only iDevices. And guess what? It’s free.