He might have been using Swap line up / down. They’re in the command palette and can be bound to hotkeys. They’re getting renamed to something else soon (maybe Move line up?), or have been in the newest version.
You should also be able to drag selected text to a new place with the mouse pointer, but it doesn’t always work for me (maybe you have to hold the pointer down for a bit before moving, I haven’t looked into it yet).