The Windows Camera app for Windows 10 Mobile has received an update this evening. And though it wasn't immediately clear what was new, the changes detailed in the Windows Insider Hub note that this update adds a slow-motion video option for select Lumia phones.
At least for now, the feature is limited to Lumia 930, 1520, and Icon phones. Users of these phones will be able to capture video at 120 fps at 720p resolution by selecting the turtle icon on the video screen. You can then view the and edit the video in the Photos app by selecting "Edit slow motion," where you'll have options to play some parts back in normal speed and others in slow-motion, along with an option for super slow motion.
If you happen to be running the Windows 10 Mobile Insider preview on a Lumia 930, 1520, or Icon, you should be able to update and check the slow-motion video out now. There's no telling if this will make it to other phones, but it's highly likely this will make an appearance on the Lumia 950 and 950 XL as well.