The first person to understand can change the teacher’s estimate of what everyone else knows.

Once one person gets it, the room appears to have got it. The teacher hears a correct answer, reads it as evidence that the explanation worked, and moves on, while everyone still lost now has an additional reason to say nothing. A single confident voice can close a topic for thirty people.

After an explanation, ask a question that requires everyone to answer rather than waiting for a volunteer. Written, or shown on fingers, anything that samples the room. The first correct answer is the least informative data point available.