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 Mission descriptions for Warhammer 40,000 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.
Most units fall back 2D6" each turn until they regroup. Some faster units, such as those on bikes or using jump packs, fall back 3D6".
A unit that falls back must move within a Fall-Back 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 so that its models are more than 2" apart, the models must be moved back into coherency as the unit moves.
Diagram 13: A unit of five models falls back from the top of the diagram toward the "Deployment table edge" arrow at the bottom. Dice showing a 4 and a 5 indicate the Fall Back roll (9" Fall Back move). Dashed arrows show each model's individual fall-back path within the "Fall Back corridor" (the lane between the unit's most extended members). Caption: "Unit must end in coherency if possible (2" or less between members) and must move within the Fall Back corridor unless it is blocked by impassable terrain."
If the unit moves into, out of, or through difficult terrain, the distance rolled on the dice that determines fall back is halved. Units will divert their Fall-Back corridor around impassable or dangerous terrain and enemy models but must move in such a way as to get back to their base line by the shortest route.