Why the students shout at the master (after meditation)?

The key practice of Zen is recovering one's awareness of one's true nature, aka Buddha nature.

When one is confused one is conflicted. One is simultaneously inhibited and at the same time has some suppressed pride and suppressed anger/aggression.

When one is enlightened one has no conflict, one is at peace, there's harmony, everything is fluid and one is authentic, sincere, true, enjoying the direct attainment of suchness.

When one practices meditation to reach from one shore to the other shore, one engages a metaphorical washing machine. This machine does the laundry, processing one's suppressed pride and suppressed anger and other such things.

At some point halfway through the journey, one's connection with one's true self recovers enough that one becomes more sincere and more authentic than before. This is when one realizes that one does not have to be suppressed anymore and can in fact let oneself loose and yell at the Zen master. This is a joyful event. The master watches very carefully and if he sees that indeed the student broke through to his true self, and is not just faking it or indulging the ego, the master approves silently, using just the eyes.

It doesn't mean yelling at one's teacher is good. What's good is breaking through from being fake and conflicted to being authentic and harmonious. A little drama during this process is okay.

Hah! I've always wondered what someone outside of Zen would think if they ever saw what went on in the dokusan room. What we have here is the Mu koan. It goes like this - "A monk asked Josshu 'Does a dog have a Buddha nature?' Josshu replies '"Muuuuuu!!!!!!!' (literally "does not; nothing; or no"). Now, of course, according to the Mahayana, everything has a Buddha nature. What, oh what, could Josshu possibly mean by that?

In order to really know what this Mu thing is, you have to sit on the cushion. You have to practice and practice deeply and sincerely sometimes for years. The gist of what you have to do is say "Mu" on your out breath. Take it all the way to the end of your exhalation. At the end of the breath, maybe you notice a little bit of space (though, trust me, at first, you won't). Over time, that little bit of space at the end of the breath becomes larger and larger. Eventually, as your smaller self starts to drop away and you descend deeper and deeper into samadhi, it will start to overtake your entire being. Then, we can say, the fun really starts. That space at the end of your breath, that emptiness, that no-thing, that Mu, will one day begin to crackle with energy with no exertion of your mind's power. When you are no longer in the way, Mu becomes a living, breathing, force that is like bottled lightening. And like bottled lightening, it starts to look for a way out. At this point, your body might begin to shake. You might feel like you want to jump up and run around the zendo. Maybe you start crowing like a rooster (that would be such a great answer!) . Mu wants to express it self. Mu wants to be seen. Mu wants to be heard. Now is the time where you present Mu to your teacher...which is what the student is doing in that video.

The really beauty of the Mu koan is that calling Mu - the act of presenting it - cannot be faked to someone who has sat in Mu themselves. We know who's putting on airs. You can hear it in their answer. Someone who is truly in the energy of Mu need only open their mouth and Mu will come pouring out. It's like turning on a faucet. You don't reach into the tap and pull out the water. The water comes out all of its own accord. Someone who is not in Mu has to supply their own energy (which is the case with the student in the video). Someone who is not in Mu may also be self conscious and block Mu from coming forth.

Don't worry if none of this makes any sense. The only way it will is if you work with Mu yourself. If you do, given enough time, I promise you that you will mingle your eyebrows with the patriarchs and speak the very same Mu that Josshu uttered more than 1200 years ago.