It isn't explained in that section of the article, but after I ask the original question, I will probe again multiple times to try to uncover the depth of a candidate's knowledge. It can be difficult with some questions because you don't want to feed the candidate the answer. The other thing is that if I were to give them feedback on the middle of the interview it could throw out there confidence for the rest of the interview (eg if say that there answer lacked depth, but they thought they did well. Or that it lacked depth because they actually didn't know any more about the topic)