Brass Knuckles are illegal fighting tools designed to improve the punching power of the wearer's fist. In a typical fight, participants form a fist by drawing the fingers across the palm and pressing the thumb against the forefinger. The flat portion between the knuckles and the first finger joints form a fairly solid weapon for striking an opponent...
The problem is that the fingers are often jammed into the fleshy part of the palm upon impact and the puncher loses most of the power behind the blow. Brute strength may still win the fight, but there's usually some serious damage to the puncher's hand.