This way it is their opinion and not one that they have gained from someone else.
Students avoid their too-outspoken peers.
I have an open door policy with my students - if my door is open they can come. I've found that many of the students, especially in my intro. Often a serious conversation in office with no audience to play to is helpful. One way is to assess how many of the class have got the concept. I tend to do this by getting them to write down what they do know.