The definition of yoga is chitta vritti nirodha. Most of the vrittis that we perform come from the inputs we gather. The biggest source of our inputs are audio and video. So, the first step is to close on them. The teacher does not intend to be a source of these. Hence he is closing the audio input through no words spoken and video inputs through the screen in betten. Every action / inaction of the teacher is aimed at imparting knowledge to his students and this is demonstrated through these conditions. I guess this is one of the many possible interpretations.