This perk unlocks the "Knife Sheath ++" trophy. An easy way to do this is at the beginning of Chapter 2, when you first get control, go left and open the door. Crouch (with circle), move forward, and pick up the extra knife. While aimed at the guard in front of you, press R3 to throw the knife for stealth kill. Press "options" for the menu, restart the checkpoint, and do the same thing four more times for a total of 5 thrown knife stealth kills (Knife Sheath + trophy) and another 15 times for the Knife Sheath ++ trophy.
To save time, you can also retrieve the knife and hunt down more guards before restarting the checkpoint. This includes a commander straight down the hall and to the right. If you continue crouching and go straight there, he'll be in position.