The victors move 2D6" forward directly after their withdrawing enemy. If they roll greater than their enemies' Fall Back score they catch and destroy them. This represents the victors shooting the enemy down as they retreat, scattering them beyond recovery, and generally jumping up and down on their corpses.
Units advancing into, out of or through difficult terrain only move half the distance rolled on 2D6.
If a unit is pursuing a foe who falls back off the board the pursuing unit will stop when it reaches the table edge.
Errata — Chapter Approved: Main Rules Clarifications (v2.0): You determine whether a unit falling back from an assault is destroyed by the pursuit based on the dice rolled not the final position of the models.
Errata — Chapter Approved: Main Rules Clarifications (v2.0): After an assault, if one side flees and the other pursues, each side rolls its dice and adjusts the score for the terrain which will be encountered during the fall back or pursuit. The modified scores are then compared to determine if the fleeing unit is caught. The actual placement of the models is never relevant.