How can I enable 240 fps slow motion video recording on the iPhone?

iPhones starting with the iPhone 6 and iPhone 6 Plus can playback slow motion video at just one eighth (12.5%) of normal speed. To achieve this effect, video is actually recorded at 240 frames per second (fps) and played back at normal speed (30 fps). While slo-mo can be recorded at 120 fps in full HD (1080p), the higher frame rate of 240 fps is captured at 720p HD resolution.