The victorious unit can choose to pursue or restrain pursuit.
If the losing unit failed its Break test, it must flee, but the winning unit now has a choice of actions. It can pursue the foe, seeking to solidify the victory by scattering or slaying the survivors of the fight. Alternatively, the victorious unit can attempt to stand fast, using the precious seconds earnt by its victory to reorder itself and prepare for other fights to come. Restraining from pursuit in this manner requires a degree of organisation and discipline that does not come easily in the heat of battle.
Accordingly, if you wish your unit to restrain pursuit, it must first take a Leadership test to see whether or not its leaders have been able to keep it in order. If the test is passed, the unit conforms to your wishes and holds position, and can choose to reform once the defeated enemy has fled. If the test is failed, then no amount of bellowing or bullying will be sufficient to keep the unit in order – they elect to pursue the enemy anyway.