After you get the reflect Cypher (from beating Solo) you can reflect bullets directly back at the enemy if you time your attacks right, do this to 10 enemies and you'll unlock this.
This is almost unmissable in the game unless you try to avoid doing it, but I got it after the first or second corridor past Solo because I was trying it out. It's a very useful function for the next section of the game, and the fastest way to take out snipers, so it should come naturally.
Note: swinging away (rapidly attack) doesn't seem to kill enemies as the bullets go in all directions, this is safer to the Strider, but you will need a well timed attack to send the bullet back to the enemy and kill him.