You are much more adept at stealing than the normal crook. You can Steal with the best of them, because with this perk, you ignore size and facing modifiers, when stealing from a person.
Fallout actually requires 80% Sneak, not Steal. This may be a bug, or the manual is incorrect.
“You have quick hands and sticky fingers, and for those favoring a stealthy method of foe disposal (or gathering items such as keys from folks without resorting to gunplay or fast-talking), this is a fine perk to take. Remember Rank 2 is the only way you're able to drop in a live grenade as a special present, and only if your adversaries haven't seen you.”— Fallout 4 Vault Dweller's Survival Guide description
Picking pockets is now twice as easy, and you can steal equipped items.
Increasing the Perception stat can increase your chance at a successful use of Pickpocket by approximately 2% per point. Heavier objects, such as most heavy armor pieces, incur a major penalty to your chances.
The heaviest equipped objects, notably the minigun and its energy variant, may not be pickpocketed as their weight reduces the chance to zero.
The maximum chance to pickpocket any object in Fallout 4 is 90%, regardless of any other perks, bonuses, or item weight modifiers.
Pickpocketing the fusion core from someone wearing power armor will cause them to exit the power armor.
In order to pickpocket equipped items, the player must be hidden from the target.
Uniforms and outfits can be "farmed" via Rank 4. So long as the target is stripped of all worn clothing (including armor), they will reset their outfit when the player re-enters the cell.
The game allows you to steal a synth uniform from certain synth NPCs in the Institute so long as they are not walking, such as when they are sweeping or acting as a merchant. These uniforms may be otherwise unobtainable even if the synth is killed.
The Rank 4 description of the Pickpocket perk can be deceiving: "twice as easy" means a 40% chance to steal with no rank is now 80%, it does not multiply from previous ranks.
All NPCs will turn hostile if they catch you pickpocketing them. This is also true for companions even if they have maximum affinity, and/or have been romanced
Equipped weapons can be pickpocketed prior to rank 3 by a two-step process. First pickpocket all of the ammunition type for the equipped weapon. This will cause the weapon to be unequipped by the NPC. On subsequent pickpocketing attempts the now-unequipped weapon will be available to steal.
Any normal item type can be placed onto the victim, at any rank, with the same chance of success as for stealing the item. Rank 2 simply adds the ability to place grenades in the live state rather than place inactive grenades.
Placing items carries exactly the same risks as stealing them, in that the NPC will turn hostile if they detect the activity. This is true even if giving them Stimpaks, other Aid, ammunition, better weapons, etc.
Placing items by pickpocketing is nonetheless the only way to transfer items to NPCs who are are not allied settlers or companions. Whether such NPC will equip or use the items transferred is dependent on the AI package of that NPC.