Fall back is a fighting withdrawal, not an out-and-out rout. Sometimes a fighting retreat in the face of overwhelming odds is the only option left. A withdrawal can give troops the chance to retire to a stronger position to mount a fresh attack or hold off the enemy.
As a unit falls back it moves directly to the player's table edge, or to the base line where the unit deployed/entered the table if it came on at a different place. We've specified this in the scenarios in the Battles Book but if you make up your own scenarios be sure you know where your units fall back to. If possible the unit must remain coherent, with gaps of no more than 2" between models as they fall back.
Troops fall back 2D6" each turn until they regroup. Particularly fast units fall back further than this but where this happens it will be noted in the special rules for them.
A unit that falls back must move within a corridor which lies between its most extended models as shown in the diagram – but individual models can move within this corridor as you wish. If a unit is spread out such that its models are more than 2" apart the models must be moved back into coherency as the unit moves.
If the unit moves into, out of or through difficult terrain the distance rolled on the dice that it falls back is halved. Troops will divert around impassable terrain, but must move in such a way as to get back to their base line by the shortest route.
Diagram: Falling Back. A Dark Eldar warrior squad falls back 8" within its fall back corridor, moving toward the friendly table edge. One of the warriors has to avoid impassable terrain as it falls back.