The verb *to slash* [comes from the Middle French esclachier][1] and means to cut with a single sweeping motion. If you hold a knife or sword and slash something in front of you like a curtain or an enemy, and you are right handed like most people, the wound will be from top right to bottom left. Think of the middle part of the Z in Zorro. 

*Back* means the other way.


  [1]: http://etymonline.com/index.php?term=slash&allowed_in_frame=0