Hobbies And Interests
Mask
You do not obtain the mask itself, but rather what it represents. After performing a stealth takedown, the enemy you killed will either drop one weapon or two. If he only drops one weapon, it will have twice as much ammunition in it. Basically, a stealth takedown rewards you with more firepower, and the blue mask is an indication that the takedown has been successful.
Clinging
When you enter a level with walls or structures, walk up to them and press the "Circle" button. This causes your character -- Nathan Drake -- to stick to the wall. You can also crouch behind a shorter wall or structure by approaching it and pressing the "Circle" button. Wait for an enemy to be near you, then press the "Square" button to perform the stealth attack: you grab your enemy from behind and choke the life out of him. After your enemy's death, the blue mask appears and the weapons dropped by the enemy or yours to keep.
Slow Walk
You can also perform a stealth attack by sneaking up behind your enemy. Lightly press the right analog stick forward. If you press it too hard, you will sprint. Your character should tip-toe his way behind an enemy. Once you are positioned behind the enemy, press the "Square" button to take him down quietly. However, if the enemy becomes aware of you, he will either take a shot at you or take a swing at you, depending on how close you are to him. If he swings, keep pressing "Square" to perform a melee attack; alternatively, run away, pull out your gun and shoot him.
Purpose
Outside of earning extra firepower -- and seeing that cool-looking mask logo -- a stealth takedown will help you save ammunition. In some situations, you will find yourself without any type of weapon, with a wave of enemies in front of you. Using stealth, you can sneak around the level, taking out the enemies one at a time, or take out one enemy with stealth and then use his weapon to take out the rest. The less you are seen by your enemies, the more of an advantage you have.