Units forced to withdraw through enemy units will be caught in a deadly crossfire and destroyed or irrevocably scattered. If a unit is withdrawing towards the enemy measure the distance to the nearest enemy model within the fall back 'corridor' before rolling to see how far the unit falls back. If the distance rolled takes the unit up to or past the enemy model the unit is destroyed.
Note that only models from units that have not fought in close combat that turn can be used for crossfire purposes.
Diagram: Crossfire. A Dark Eldar squad falls back down its "Fall back corridor" toward the top of the diagram, but a Space Marine model stands inside that corridor 6" from the nearest Dark Eldar model. Dice show a "Fall back roll" of 4. Caption: "A Dark Eldar squad is falling back but a Space Marine is inside their fall-back corridor. He is 6" from the nearest Dark Eldar, so if the warriors roll a 6 or more on 2D6 they will be destroyed. They roll a 6 and are killed in the lethal crossfire."