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 corridor which lies between its most extended models — 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.
Diagram: Illustration of the fall-back "corridor" formed between a falling-back unit's most extended models, within which individual models may move freely while retreating.
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 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.
Firing While Falling Back
Troops who are falling back may continue to shoot, but as they are moving they may not fire heavy weapons, or rapid fire weapons at full effect. They also cannot launch an assault.
Assaulted While Falling Back
A unit which is assaulted by the enemy whilst falling back must check to regroup immediately (see the core rulebook's Regrouping rules). No modifiers apply to the check in this case, and even units which would not normally be allowed to regroup are allowed to make the check – the situation is sufficiently life-or-death threatening for the usual regrouping restrictions to be ignored. If successful the unit is regrouped and will fight the close combat normally. If it fails to regroup the unit is scattered and counts as destroyed.
Trapped!
Sometimes a unit will find its fall-back corridor blocked by impassable terrain and/or enemy models. Ignore enemy models that have fought in close combat against the unit this turn. A unit falling back may move around any obstruction in such a way as to get back to their baseline by the shortest route. If a unit cannot perform a full fall back move in any direction, without doubling back, entering impassable terrain or coming within 1" of the enemy, it is destroyed.
Note: This replaces the old crossfire rules.