Moderators can append replies to questions that have been submitted through Social Q&A. In most cases, the speaker isn't going to be adding replies to questions during the live presentation but if someone else (who's not on stage or presenting) is acting as a moderator, they can be answering / responding to questions in real-time.

To append an answer simply click on the question and type in your reply.

Attendees will see a notification that a moderator answer has been added to that question, and they can simply tap on the question to view the response.