The iPhone 5s introduces a new Slo-Mo camera mode, in which you can create videos that seamlessly slide from normal video to super-smooth slow-motion action and back again. That smooth motion happens because in Slo-Mo mode, the iPhone 5s is shooting video at 120 frames per second (instead of the usual 30), so when it's running at a quarter-speed, it's still running at the same frame rate as regular video. There's no stutter of repeated video frames that you'd see in a fake slow-motion effect.
On the iPhone 5s itself, the Camera app can let you quickly make and share (albeit with some complications) slow-motion videos. But back on your Mac, things get a little more interesting.
Copy a Slo-Mo video back to your Mac and you get a normal QuickTime movie file. Open it up in QuickTime Player and you'll see a regular old video playing back at regular speed--no slow-motion at all, even if you set in and out points on your iPhone 5s.
If you look closely, though, you'll notice that everything seems a little smoother than a normal video does. That's because you're watching that video play back at 120 frames per second. All the information necessary to make a slow-motion video is there--it's just all playing back at normal speed rather than one-quarter speed.
The good news is, you can import this video into iMovie (or Final Cut Pro, or any other video editor you can name) and create your own slow-motion effect. The Camera and Photos apps on the iPhone 5s will only let you create a single slow-motion region; in a video editor you can flip into and out of slow-motion mode as many times as you want.
In iMovie, just import the movie file as you would any other iPhone video. you'll see that the thumbnails are tagged with a small icon in the bottom-left corner that reads "120," because it's a 120 frames-per-second video.
To emulate the iPhone's video effect in iMovie, add the clip to your iMovie project and then split it into three parts--the beginning, the middle part (which you'll switch into slow motion), and the ending. Then select the middle part and choose Clip > Slow Motion > 25%. You'll have to give iMovie a few seconds to optimize the clip, and that's it.
Using this approach, you can shoot a long high frame-rate sequence using the Slo-Mo mode and then create a video that switches into and out of slow motion as often as you like. Imagine a diver slowly jumping off the diving board, then moving at full speed until just before hitting the water. Or a skateboarder who slows down every time she's airborne.
Here is a video we shot on the iPhone 5s but edited in iMovie on the Mac.