The victorious unit can choose to pursue or restrain pursuit. a fleeing and pursuing unit both move 2D6".
If the losing unit failed its Break test, it must flee, but the winning unit can choose to either pursue the fleeing unit or restrain.
I you wish your unit to restrain pursuit, it must first take a Leadership test. If the test is passed, the unit can choose to reform once the defeated enemy has fled. If the test is failed, the unit is forced to pursue.