This, like all perks, can be found by pressing the Back button and using LB/RB to get to Perks.
This particular one is the second perk in the Stealth category, and requires 5 silent takedowns. Crouch (B) and sneak up behind an enemy, then press RS when you're close enough to take him down. I don't think that silenced pistol kills will count for this.
Doing this will unlock the ability to throw knives for other silent kills.