This is a fairly random event that happens every so often while attacking enemies with your axe. As you swing, sometimes you will perform a kill that cuts off the enemies head. this can be the first hit, or a following hit, but it always results in the enemy dying by a beheading. There are several enemies throughout the game that have their back to you, usually if you use your axe to kill them it will usually be with a single shot kill. This will unlock at some point, but as it's semi random it's hard to perform a 1 hit kill every time.