Instructions
Visit the character called "Popcorn" to purchase counter kill moves from him. You will find him in the abandoned theater across the street from the main character's apartment.
Use the left analog stick on the PSP to approach the enemy you want to perform a counter kill on. If the enemy is wielding a gun, do not go for a direct approach or you will get injured. Flank him, or approach him from behind.
Press the "Square" button once you are right next to the enemy. The screen will switch to a short animation showing your character performing the counter kill. These are random and taken from the counter kills you have available and also depend on the kind of weapon the enemy is carrying.