Assuming that you've installed the 'Photos Add-ins' application* and that you're bang up to date with your Windows 10 Mobile Store applications, here's what you should see happen:
* Yes, the name does seem inconsistent in some places, I guess Microsoft is still working on naming!
Lumia Moments has been quietly renamed and improved by Microsoft and here's the new version, neatly installing itself into Windows 10 Mobile Photos - all captured videos now have this headline function, in addition to usual sharing, 'editing' and deleting controls.
The usual choice to pick a best frame (e.g. an 8MP still from 4K video) or try one of the fancy multi-frame action shots.
The interface has been tightened slightly, the timeline and swipe-for-best-frame are now concurrent - you can use either, depending on how long the video clip is. Very slick indeed.
The name's especially interesting because it does imply that there will be other 'Add-ins' in the future. Watch this space!
Good stuff, and a theme I'll return to in an editorial very soon.