See these short dialogues as illustrations. "He's a good prime minister." "How so?" (meaning "In what way is he good?") "I want to give my mother a Formula 1 car." "What for?" (meaning, "For what purpose? Does she need it?") "I want to quit my job." "Why?" (meaning, "What's the reason?")

"How so?" basically means "Can you explain what you mean by that?"

Go with you gut feeling.