Take a photo with Live Photo's turned on, then view it.
Swipe up on the picture and a hidden menu of features appears!
Way over to the right of it is "Long Exposure"
It works really well on moving water, like waves, fountains or waterfalls. Creating a composite of all the frames taken in Live Photo mode, just like taking a long exposure with a camera, where the shutter is open for longer periods of time.
You just need to be nice and still when taking the initial shot, or else you lose some of the sides, top and bottom as it crops into it, taking into account your movement.
Personally, I think that Apple should have plugged the hell out of this feature that actually works, instead of the Portrait Modes which are still hit and miss!