The Zen koans begin with the Buddha on Spirit Vulture Peak imparting his awakening to Mahākāśyapa if not also checking him out to see if they were seeing eye to eye. How did the Buddha do this? He simply held up a flower then blinked.
In response to the Buddha’s strange behavior, Mahākāśyapa just smiled. But why did Mahākāśyapa smile? Why not say something? Apparently, Mahākāśyapa understood what the Buddha was up to. He got the message.
For those reading this koan for the first time this seems strange and puzzling. Something is missing. Why does one hold up a flower and blink and the other just smile? We are not aware of what connects the two which makes up the mysterious Mind to Mind transmission of Zen.
No matter how much our intellect tries to make sense of this, it cannot. But Zen is not so stupid as to expect us to find some kind of hidden intellectual solution because what connects the two and gives rise to their strange responses transcends our all-too-human world with its grasping intellect.
The unraveling of the koan[s] or "barriers" comes with kenshō (intuiting one's nature). This one with Mahākāśyapa is no different . Every single koan without exception has been composed by a person who is awakened. You are expected to do the same, that is, awaken!
What both the Buddha and Mahākāśyapa have in common in this koan is the Mind that is luminous (prabhāsvara) which is only met with when one actually awakens. It doesn’t matter which koan is solved.
The heart of Zen is in this spiritual transmission even though, in all probability, it never actually took place. It's a Zen myth, but an excellent one that helps us to engage with the mystery of Mind which is what animates our temporal bodies. It is the Mind that the Buddha in the Pali Nikayas calls the luminous Mind. It is also the Mind that is awakened or bodhicitta.
It's rather a well kept secret that Buddhist teachers, who are worth their salt, don't discuss this luminous Mind too much. They understand that when the adept engages with this Mind, it is self-verifying and clearly reveals what was actually transmitted between the Buddha and Mahākāśyapa. This, by the way, is the orthodox transmission which means that when we awaken to Mind, we are transmitted. We also join theTriple Gem Sangha.