A diadem is a symbol of authority. By knocking off Arjuna’s diadem, Karna expresses his long-simmering jealousy toward Arjuna’s higher status. This insult prompts Arjuna to respond with an even stronger attack, proving that, for all Karna’s prowess in battle, he can’t compete with Arjuna, particularly not when he has the power of Krishna on his side. In this passage, Krishna literally steers Arjuna to safety, and this guidance is symbolic of how Krishna steers Arjuna to make choices according to dharma.