I think we get the pain token if any of the enemy models touch the edge of the table when they fall back. From that point the enemy unit is considered "destroyed". We do not get a pain token if the unit falls back but remains on the table.
We can also trap them by cutting off the escape route by fencing the enemy between impassable terrain and our unit, disallowing them from completing the fall back move. Trouble is, impassable terrain is something that has never featured in any of my games yet and is probably pretty rare.
Kia Kaha, Dark Eldar players!