Hi Messyhead, Stable-Hover sets the Hover height and maintains that height. Alt-J and Alt-K adjust the Hover height down/up respectively. Watch your altitude gauge as you press the keys, it jogs up or down 10 ft or 5 meters depending upon whether you are blue or red.
Pop-Up works Up and Down from the Hover height. Pop-Up toggles Up 20m on the first keypress, then Down 20m on the second keypress.