Zen is famous for koans (called kong-ans in Korean, and in this book), those bizarre and seemingly unanswerable questions Zen masters pose to their students to check their realization (such as What is the sound of one hand clapping?).
Fear of koans keeps some people from ever giving Zen practice a try.
But here, through the experience of seeing a modern Zen master work with his students, you can see what koan training is really like: It's a skillful, lively practice for attaining wis.