We form echo tags questions like ordinary question tags, but they are said in reply to a statement. They usually show interest and encourage the speaker to continue, but they can also express other meaning depending on the intonation.
Affirmative statements usually have positive echo tags, and negative statements usually have negative echo tags. With a rising intonation we are showing interest and the conversation will usually continue.
He left. He did, didn’t he? (confirmation)
He left. Did he? (surprise)
He didn’t come. He didn’t, did he? (confirmation)
He didn’t come. He didn’t? (surprise)