let me shed some light on this little action,
those 3 selected tiles are the ones that you move a unit to, to make this effect, place one of these tiles next to unwalkable terrain, create or move a unit on the null, then order the unit to move to the unwalkable terrain. Now, you can order them to move, or click REALY fast to make them move were you want, but you will always have that uncontrolable movement, also if the unit leaves the unwalkable terrain, it can't go back on it unless you move it back to the null
NOTE: you can't move the unit onto the null manualy, you must use the teleport(move) unit to location(null)