This happens when you vault over cover (using ), and an enemy is on the other side of it. This will make your character kick the enemy. This may be hard to get on the harder difficulties as most of your battles will take place at range, but there are a few places you will be able to get it. You can always revisit chapters on lower difficulties once you beat the game.
A notable place to get this is during Chapter 10. There will be a cutscene where one of the bad guys has one of your partners at gun point. After the cutscene, go forward and you will see a soldier on the other side of a barricade with his back to you. Mount the barricade, press to vault over, and you will kick him right over the edge. The level is set-up specifically for you to do this.