Violent koans aren't particularly common, they just stand out. However, they do reflect a fundamentally destructive aspect of spiritual practice, namely, that

Violent koans aren't particularly common, they just stand out. However, they do reflect a fundamentally destructive aspect of spiritual practice, namely, that it undermines the conditioning behind cherished identities. The point of the story is that the boy was chasing after validation as a Zen master and had adopted the finger-raising ritual as a part of his identity as a competent Zen practitioner. When Gutei cut it off, he was removing a support for a cherished identity, and this was the basis for the boy's identity at that moment.