Through sheer force apparently. (they don’t like fighting higher strength ranked enemies. I heard the Skaven actually make great allies in WH 3 Immortal Empires if you are both stronger than them and stronger than most enemy factions, they will chicken out if the enemy faction is stronger than you. I however have not tested this out.)
Also Reggie makes this joke during his play through of Clan Moulder (nearly got his ass beat by Karl Franz)