What is "mordhau"

In the German school of swordsmanship, Mordhau, alternatively Mordstreich or Mordschlag (Ger., lit., "murder-stroke" or "murder-strike" or "murder-blow"), is the technique of holding the sword inverted, with both hands gripping the blade, and hitting the opponent with the pommel or crossguard. This technique allows the swordsman to essentially use the sword as a mace or hammer. The Mordhau is mainly used in armoured combat, although it can be used to surprise an opponent in close quarters. This technique has also been called a "thunder stroke".