Knuckles and some cards are used to Scare Minions. This is different from Defeating a Minion.

You Defeat a Minion by matching your attack icons to their colors. You then gain money and skills and take Damage from biting Minions.

You Scare a Minion by using Knuckles or Special cards to remove it from the board. You then put it in a discard pile and receive no money or skills and take no bites. (Some cards break this rule and give you rewards for Scaring Minions.)