The Rinzai School of ZEN established an approach to teaching the Art of ZEN based primarily on the cultivation of the final insight into the Fabric of Reality by means of an experiential exercise, utilizing so-called Koans and limited book learning (see also my post There is No Spoon-Revisited for further details on the practice of Koan Meditation). If metaphors, intellectual or dualistic descriptions or other Expedient Means based on words and language had to be utilized in pointing at Ultimate Wisdom, they tended to be harsh if not to say brutal in their application. Huang Po for example is known to slap a Divine Emperor of the T’ang Dynasty during a personal reception, apparently because he failed to display the correct attitude towards the understanding of ZEN as taught at the time. John Blofeld’s comment to this episode as recounted in his translation of The ZEN Teaching of Huang Po goes as follows:
Discover teachings of the Zen Buddhism's prominent teacher Huang Po. His teachings translated and published in the book "The Zen Teaching of Huang Po On the Transmission of Mind" by John Blofeld offer spiritual teachings of the highest purity.
You'll find them invaluable, on all stages of your way to Realization. Reaching Enlightenment according to Huang Po is possible now, and everyone can succeed. Be one of those who do!