Brass in Allomancy
When burning brass, an Allomancer can target either a single person or a group of people. Additionally, the Allomancer can target specific emotions to dampen. For example, if an Allomancer burning brass wanted a soldier to flee, he would Soothe away all of his emotions except for fear and panic. Though this would make the soldier more likely to flee, it would not force him to do so; a sufficiently brave, stubborn, or aware individual could choose to stay.
Mistings who can burn brass are called Soothers. Though they are viewed with suspicion by many, brass does not grant an allomancer mind control. Similarly, burning brass does not give an allomancer any insight ot the emotional state of the target.
Basic strategy while using brass in Allomancy, or Soothing, is usually confined to Mistings who burn it. This is because Mistborn, who can burn all the metals in Allomancy, are usually found in battles. Now in battle, a swift jerk on the emotional chain could cause panic or confusion, no strategy is really called for. If a Mistborn had the time and ability, they could do this in battle, but such touches require finesse. Not to mention, if two Mistborn are fighting, it is unlikely that either of them are not burning copper.
This is the opposite when concerning Soothers. Being able to read and then covertly manipulate the emotions of others is often a long term but quiet battle. Usage of these powers is more often applied in "mental" combat and not physical combat. If a Soother were to blatantly Soothe away emotions, a logical person knew they should be feeling something that they don't and may guess at what is happening. For this very reason it is important that a competent Soother can understand how another is feeling and have a light enough touch to not be noticed when they are pushing on someone else's feelings.
Brass in Feruchemy
When used in Feruchemy, brass will store warmth. To achieve this, the Feruchemist must spend time being very cold, siphoning off the body's own warmth into a brass metalmind. The Feruchemist can then tap the metalmind at a later date, gaining warmth.
Brass in Hemalurgy
Brass steals Feruchemical mental attributes when used in Hemalurgy.
In the Final Empire, those few people who knew about Feruchemy assumed that the Steel Ministry hunted Terris Keepers because the Lord Ruler wanted to wipe out the ability in their race. However, the Canton of Inquisition needed Feruchemists; without them, they could not make Inquisitors.